[llvm] [PostRASink] Add target hook shouldPostRASink (PR #167182)

[Junjie Gu via llvm-commits]

jgu222 wrote:

> LLVM Buildbot has detected a new failure on builder `mlir-nvidia-gcc7` running on `mlir-nvidia` while building `llvm` at step 7 "test-build-check-mlir-build-only-check-mlir".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/116/builds/20727
> 
> Here is the relevant piece of the build log for the reference

This PR has no functional impact and futhermore, this mlir test does not involve codegen yet (still mlir). This PR does not cause this lit failure.  The failure must have been due to some other reason.

https://github.com/llvm/llvm-project/pull/167182