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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303100 91177308-0d34-0410-b5e6-96231b3b80d8
