Tidy up CMake style across the project. (#9000)

This change _should_ be an NFC, just going through and aligning our CMake files on a consistent style for items like:

* No spaces between commands and the opening parenthesis: use `if(...)`, instead of `if (...)`
* No variable references when not needed: use `if(VARIABLE_NAME)` / `if(NOT VARIABLE_NAME)`, instead of `if(${VARIABLE_NAME})` / `if(NOT ${VARIABLE_NAME})`
* Local variables use `_UPPER_CASE`, instead of `_lower_case` or `UPPER_CASE`
* Parsed arguments use the `_RULE` prefix, not `ARG` (LLVM mostly uses `ARG`, but we've been using `_RULE`)

Individual commits have the details for how they were generated (surprise surprise, it's a pile of regex find/replaces with some manual cleanup sprinkled on top), as well as the rationale where applicable. I may have missed some cases in my search, and we have no automated formatting or enforcement.

---

Inspired by https://github.com/google/iree/pull/8933#discussion_r853526858:

> Use if(IREE_ENABLE_CPUINFO)
> (we're super inconsistent on this but I try to get it right going forward)
diff --git a/build_tools/cmake/iree_python.cmake b/build_tools/cmake/iree_python.cmake
index 64e9143..0dc833b 100644
--- a/build_tools/cmake/iree_python.cmake
+++ b/build_tools/cmake/iree_python.cmake
@@ -70,27 +70,26 @@
     "COMPONENT;PACKAGE_NAME;MODULE_PATH"
     "DEPS;ADDL_PACKAGE_FILES;FILES_MATCHING"
     ${ARGN})
-  set(_install_component ${ARG_COMPONENT})
-  set(_install_packages_dir "python_packages/${ARG_PACKAGE_NAME}")
-  set(_install_module_dir "${_install_packages_dir}/${ARG_MODULE_PATH}")
-  set(_target_name install-${_install_component})
+  set(_INSTALL_COMPONENT ${ARG_COMPONENT})
+  set(_INSTALL_PACKAGES_DIR "python_packages/${ARG_PACKAGE_NAME}")
+  set(_INSTALL_MODULE_DIR "${_INSTALL_PACKAGES_DIR}/${ARG_MODULE_PATH}")
 
   if(NOT FILES_MATCHING)
-    set(_files_matching PATTERN "*.py")
+    set(_FILES_MATCHING PATTERN "*.py")
   else()
-    set(_files_matching ${ARG_FILES_MATCHING})
+    set(_FILES_MATCHING ${ARG_FILES_MATCHING})
   endif()
 
   install(
     DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/
-    COMPONENT ${_install_component}
-    DESTINATION "${_install_module_dir}"
-    FILES_MATCHING ${_files_matching}
+    COMPONENT ${_INSTALL_COMPONENT}
+    DESTINATION "${_INSTALL_MODULE_DIR}"
+    FILES_MATCHING ${_FILES_MATCHING}
   )
 
-  set(PY_INSTALL_COMPONENT ${_install_component} PARENT_SCOPE)
-  set(PY_INSTALL_PACKAGES_DIR "${_install_packages_dir}" PARENT_SCOPE)
-  set(PY_INSTALL_MODULE_DIR "${_install_module_dir}" PARENT_SCOPE)
+  set(PY_INSTALL_COMPONENT ${_INSTALL_COMPONENT} PARENT_SCOPE)
+  set(PY_INSTALL_PACKAGES_DIR "${_INSTALL_PACKAGES_DIR}" PARENT_SCOPE)
+  set(PY_INSTALL_MODULE_DIR "${_INSTALL_MODULE_DIR}" PARENT_SCOPE)
 endfunction()
 
 # iree_pyext_module()
@@ -204,7 +203,7 @@
 # PYEXT_DEPS: List of deps of extensions built with iree_pyext_module
 function(iree_py_library)
   cmake_parse_arguments(
-    ARG
+    _RULE
     ""
     "NAME"
     "SRCS;DEPS;PYEXT_DEPS"
@@ -213,36 +212,36 @@
 
   iree_package_ns(_PACKAGE_NS)
   # Replace dependencies passed by ::name with ::iree::package::name
-  list(TRANSFORM ARG_DEPS REPLACE "^::" "${_PACKAGE_NS}::")
+  list(TRANSFORM _RULE_DEPS REPLACE "^::" "${_PACKAGE_NS}::")
 
   iree_package_name(_PACKAGE_NAME)
-  set(_NAME "${_PACKAGE_NAME}_${ARG_NAME}")
+  set(_NAME "${_PACKAGE_NAME}_${_RULE_NAME}")
 
   add_custom_target(${_NAME} ALL
-    DEPENDS ${ARG_DEPS}
+    DEPENDS ${_RULE_DEPS}
   )
 
   # Symlink each file as its own target.
-  foreach(SRC_FILE ${ARG_SRCS})
-    # SRC_FILE could have other path components in it, so we need to make a
+  foreach(_SRC_FILE ${_RULE_SRCS})
+    # _SRC_FILE could have other path components in it, so we need to make a
     # directory for it. Ninja does this automatically, but make doesn't. See
     # https://github.com/google/iree/issues/6801
-    set(_SRC_BIN_PATH "${CMAKE_CURRENT_BINARY_DIR}/${SRC_FILE}")
+    set(_SRC_BIN_PATH "${CMAKE_CURRENT_BINARY_DIR}/${_SRC_FILE}")
     get_filename_component(_SRC_BIN_DIR "${_SRC_BIN_PATH}" DIRECTORY)
     add_custom_command(
       TARGET ${_NAME}
       COMMAND
         ${CMAKE_COMMAND} -E make_directory "${_SRC_BIN_DIR}"
       COMMAND ${CMAKE_COMMAND} -E create_symlink
-        "${CMAKE_CURRENT_SOURCE_DIR}/${SRC_FILE}" "${_SRC_BIN_PATH}"
+        "${CMAKE_CURRENT_SOURCE_DIR}/${_SRC_FILE}" "${_SRC_BIN_PATH}"
       BYPRODUCTS "${_SRC_BIN_PATH}"
     )
   endforeach()
 
   # Add PYEXT_DEPS if any.
-  if(ARG_PYEXT_DEPS)
-    list(TRANSFORM ARG_PYEXT_DEPS REPLACE "^::" "${_PACKAGE_NS}::")
-    add_dependencies(${_NAME} ${ARG_PYEXT_DEPS})
+  if(_RULE_PYEXT_DEPS)
+    list(TRANSFORM _RULE_PYEXT_DEPS REPLACE "^::" "${_PACKAGE_NS}::")
+    add_dependencies(${_NAME} ${_RULE_PYEXT_DEPS})
   endif()
 endfunction()