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

[Wang Pengcheng via llvm-commits]

wangpc-pp wrote:

> > And this can be used in #77461.
> 
> How will this be used in that patch?

We only need to mark fusible pairs as `Fusible` in `MIFlag` in pre-ra scheduling. We don't care about it after RA.

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