[llvm] [RegisterCoalescer] Fix SUBREG_TO_REG handling in the RegisterCoalescer. (PR #96839)

[Kai Luo via llvm-commits]

bzEq wrote:

> I would like to do this but the test actually hits an assert so we don't actually generate any code for the X86 test unless I remove the -verify-coalescing option which shows the actual failure.

We can include error messages in the pre-commit case to show where the problem is. Something like
```
not --crash llc .... | FileCheck ...
; CHECK: <error information>
```

https://github.com/llvm/llvm-project/pull/96839