Add statistics to Util/Transforms/FoldGlobals.cpp. (#7535)

Following up on https://github.com/google/iree/pull/7312#discussion_r728261108, this adds more [statistics](https://mlir.llvm.org/docs/PassManagement/#pass-statistics) to IREE's compiler passes. I personally build with the CMake option `-DLLVM_FORCE_ENABLE_STATS=ON` so the `-pass-statistics` flag works in `RelWithDebInfo` builds, but we don't yet have anything like that configured on our CI or recommended in our developer documentation. See also https://github.com/google/iree/issues/6161#issuecomment-912635307 for how this could be useful.

Sample output on [bert_encoder_unrolled_fake_weights.mlir](https://github.com/google/iree/blob/main/iree/test/e2e/models/bert_encoder_unrolled_fake_weights.mlir) (fake weights are folded aggressively 😛):

```
mlir::iree_compiler::IREE::Util::`anonymous-namespace'::FoldGlobalsPass
  (S) 1113 global ops before folding - Number of util.global ops before folding
  (S)    7 global ops after folding  - Number of util.global ops after folding
```

iree/compiler/Dialect/Util/Transforms/FoldGlobals.cpp

diff --git a/iree/compiler/Dialect/Util/Transforms/FoldGlobals.cpp b/iree/compiler/Dialect/Util/Transforms/FoldGlobals.cpp
index 94d5b26..10b012c 100644
--- a/iree/compiler/Dialect/Util/Transforms/FoldGlobals.cpp
+++ b/iree/compiler/Dialect/Util/Transforms/FoldGlobals.cpp
@@ -31,6 +31,11 @@
 namespace Util {
 namespace {
 
+template <typename R>
+static size_t count(R &&range) {
+  return std::distance(range.begin(), range.end());
+}
+
 struct Global {
   size_t ordinal = 0;
   IREE::Util::GlobalOp op;
@@ -356,6 +361,9 @@
 class FoldGlobalsPass
     : public PassWrapper<FoldGlobalsPass, OperationPass<mlir::ModuleOp>> {
  public:
+  explicit FoldGlobalsPass() = default;
+  FoldGlobalsPass(const FoldGlobalsPass &pass) {}
+
   StringRef getArgument() const override { return "iree-util-fold-globals"; }
 
   StringRef getDescription() const override {
@@ -382,6 +390,7 @@
     FrozenRewritePatternSet frozenPatterns(std::move(patterns));
 
     auto moduleOp = getOperation();
+    beforeFoldingGlobals = count(moduleOp.getOps<IREE::Util::GlobalOp>());
     for (int i = 0; i < 10; ++i) {
       // TODO(benvanik): determine if we need this expensive folding.
       if (failed(applyPatternsAndFoldGreedily(moduleOp, frozenPatterns))) {
@@ -429,7 +438,15 @@
 
       if (!didChange) break;
     }
+
+    afterFoldingGlobals = count(moduleOp.getOps<IREE::Util::GlobalOp>());
   }
+
+ private:
+  Statistic beforeFoldingGlobals{this, "global ops before folding",
+                                 "Number of util.global ops before folding"};
+  Statistic afterFoldingGlobals{this, "global ops after folding",
+                                "Number of util.global ops after folding"};
 };
 
 }  // namespace