RegisterCoalescer: Don't delete IMPLICIT_DEF if it's live into the same block

Live out implicit_defs need to be kept, but the check for this only
checked if the block parent was the same. This doesn't work if the
parent blocks are the same but the value is live. Fixes verifier error
"Instruction ending live segment doesn't read the register", which
would appear at the coalesced non-implicit_def def.

Fixes #38788

https://reviews.llvm.org/D158882