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

[Stefan Pintilie via llvm-commits]

stefanp-ibm wrote:

> I think we should pre-commit the X86 test case to show where failure occurs.

I can do this once the reviewers are happy with the test cases. For the X86 testcase I will probably just use `-enable-subreg-liveness=true` so that it actually passes and then set it to true when this patch goes in.

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