Fix DSE miscompile when store is clobbered across loop iterations

DSE would mistakenly remove store (2):

  a = calloc(n+1)
  for (int i = 0; i < n; i++) {
    store 1, a[i+1] // (1)
    store 0, a[i]   // (2)
  }

The fix is to do PHI transaltion while looking for clobbering
instructions between the store and the calloc.

Reviewed By: efriedma, bjope

Differential Revision: https://reviews.llvm.org/D68006