RegisterCoalescer: Correctly set valid lanes when keeping live out implicit defs

This fixes some verifier errors when live out implicit defs are
coalesced with identity copies. Fixes some reduced testcases from
issue #38788 but doesn't solve the original failure.

I was surprised this seems to obviate the special casing in
analyzeValue that's been there since the subregister liveness support
went in.

https://reviews.llvm.org/D158850