[CVP] simplify phi with constant incoming values that match common variable edge values

This is based on an example that was recently posted on llvm-dev:

void *propagate_null(void* b, int* g) {
  if (!b) {
    return 0;
  }
  (*g)++;
  return b;
}

https://godbolt.org/g/xYk3qG

The original code or constant propagation in other passes has obscured the fact 
that the phi can be removed completely.

Differential Revision: https://reviews.llvm.org/D45448

llvm-svn: 329755