[llvm] [MacroFusion] Add IsPostRA to indicate whether running in post-ra scheduler (PR #77567)

[Craig Topper via llvm-commits]

topperc wrote:

> And this can be used in https://github.com/llvm/llvm-project/pull/77461.

How will this be used in that patch?

https://github.com/llvm/llvm-project/pull/77567