[clang] [CIR] Add TargetLowering pass (PR #179245)

[Sirui Mu via cfe-commits]

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@@ -3339,7 +3315,8 @@ void ConvertCIRToLLVMPass::runOnOperation() {
   mlir::ModuleOp module = getOperation();
   mlir::DataLayout dl(module);
   mlir::LLVMTypeConverter converter(&getContext());
-  std::unique_ptr<cir::LowerModule> lowerModule = prepareLowerModule(module);
+  std::unique_ptr<cir::LowerModule> lowerModule =
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Lancern wrote:

No, `LowerModule` should no longer be needed in `LowerToLLVM`.

But I plan to make this PR as simple as possible. Removing `LowerModule` from `LowerToLLVM` requires non-trivial amount of work (e.g. you have to update the TableGen backend as well because `LowerModule` is included in every `CIRToLLVMXxxOpLowering` pattern) so I decide to do it in a later PR.

https://github.com/llvm/llvm-project/pull/179245