[Codegen][CPU] Wire LowerBitcodeUKernels into the Mmt4dTilingExpert pipeline. (#24570)

Completes the new-style C-ukernel pipeline integration:

* `LLVMCPUTarget.cpp`: install `iree_codegen.ukernel_provider =
#iree_cpu.ukernel_provider` on every LLVMCPU executable target config
whenever `--iree-llvmcpu-enable-llvm-ukernels` is non-empty. This
mirrors what ROCM does and is what lets the generic
`LowerBitcodeUKernelsPass` find the CPU provider for the
bitcode-lookup-and-attach.

* `KernelDispatch.cpp::setRootConfig(InnerTiledOp)`: when
`selectUKernel` returns a descriptor, set the reduction-dim vector tile
size to 0 (untiled) instead of 1 (unrolled). The ukernel's C function
takes the outer K extent as a runtime argument (`k_outer`) and runs the
K loop itself; pre-unrolling here would both defeat that and bloat the
IR. Matches the equivalent split in
`setDataTiledMmaInnerTiledLoweringConfig` on the GPU side.

* `Passes.cpp::addMmt4dTilingExpertPassPipeline`: insert
`createLowerBitcodeUKernelsPass()` between the parallel-tile and
reduction-tile passes. A nop when no op carries a `iree_codegen.ukernel`
descriptor, so it stays in the default pipeline. The companion
`LowerUKernelOpsToCalls` already runs after bufferization (in
`addLowerToLLVMPasses`); together they implement the user's "early
`inner_tiled` -> `ukernel.generic` on tensors; late `ukernel.generic` ->
`func.call` on memrefs" split.

Lit test (`select_ukernel.mlir`) gains a second RUN line that chains
`iree-llvmcpu-select-lowering-strategy` with
`iree-codegen-lower-bitcode-ukernels` and asserts:
- the `inner_tiled` is rewritten to `iree_codegen.ukernel.generic`
carrying the right `iree_uk_mma_x86_avx512bf16_1x16x2_f32_bf16` name;
- the matching bitcode appears as `hal.executable.objects` (resolved
from the global `EmbeddedDataDirectory` populated at LLVMCPU plugin
init);
- and the original `inner_tiled` is gone, guarding against the rewrite
silently no-op'ing on a regression.

An end-to-end `iree-compile` test would also be desirable, but the
current pipeline state needs more wiring (workgroup-distribution
verifier complains about the bufferization-introduced copy ops around
the workgroup loop, on both the ukernel and non-ukernel paths) — that
investigation belongs with the follow-up commit that fills in real
ukernel bodies and an actual numerical matmul test.

Progress towards https://github.com/iree-org/iree/issues/24574.

Signed-off-by: Benoit Jacob <jacob.benoit.1@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>