[PHIElimination] Lower a PHI node with only undef uses as IMPLICIT_DEF

Summary:
The lowering of PHI nodes used to detect if all inputs originated
from IMPLICIT_DEF's. If so the PHI node was replaced by an
IMPLICIT_DEF. Now we also consider undef uses when checking the
inputs. So if all inputs are implicitly defined or undef we
lower the PHI to an IMPLICIT_DEF. This makes
PHIElimination::LowerPHINode more consistent as it checks
both implicit and undef properties at later stages.

Reviewers: MatzeB, tstellar

Reviewed By: MatzeB

Subscribers: jvesely, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D52558

llvm-svn: 343417