[llvm] [AMDGPU] Fix for 131386 by reducing implicit definitions on register restoration (PR #133986)

[Ryan Buchner via llvm-commits]

bababuck wrote:

Updated with the following changes:
- Added new MIR test that reduces the failing scenario into a few lines of MIR code
- Update the source code change to only change the if statement (no formatting changes to the surrounding code)
- Rather than dropping `IsLastSubReg`, I instead qualify it with checking that the register state is not a define. I noticed in the new test I wrote that an `implicit-kill` was being dropped after my original change.

https://github.com/llvm/llvm-project/pull/133986