[llvm] [CodeGen][RegCoalescer] Preserve valid lanes when keeping IMPLICIT_DEF (PR #215272)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

This seems OK. Can you update the description in the summary? The one in the test is better but somewhat hidden away.

https://github.com/llvm/llvm-project/pull/215272