[PHIElimination] Fix the killed flag for LowerPHINode()

Summary:
In the phi-node-elimination pass, we set the killed flag incorrectly.
When we eliminate the PHI node, we replace the PHI with a copy for the
incoming value.

Before this patch, we will set incoming value as killed(PHICopy). And
we will remove the killed flag from last using incoming value(OldKill).
This is correct, only if the new PHICopy is after the OldKill.

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D80886