[llvm] [LivePhysRegs] Make use of `MBB.liveouts()` (semi-NFC) (PR #154728)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> The following buildbots are also broken after this patch
> 
> https://lab.llvm.org/buildbot/#/builders/197/builds/8321 https://lab.llvm.org/buildbot/#/builders/211/builds/1549
> 
> Please take a look and fix ASAP.

Will do :+1: It looks like MSVC does not like the cast to `MCRegister`. 

https://github.com/llvm/llvm-project/pull/154728