[PHIElimination] Update LiveVariables after handling an unspillable terminator

Update the LiveVariables analysis after the special handling for
unspillable terminators which was added in D91358. This is just enough
to fix some "Block should not be in AliveBlocks" / "Block missing from
AliveBlocks" errors in the codegen test suite when machine verification
is forced to run after PHIElimination (currently it is disabled).

Differential Revision: https://reviews.llvm.org/D110939