[NewGVN] Fix verification of MemoryPhis in verifyMemoryCongruency().

verifyMemoryCongruency() filters out trivially dead MemoryDef(s),
as we find them immediately dead, before moving from TOP to a new
congruence class.
This fixes the same problem for PHI(s) skipping MemoryPhis if all
the operands are dead.

Differential Revision:  https://reviews.llvm.org/D33044

llvm-svn: 303100