Handle forwarding local memsets to loads. For example, we optimize this:

Handle forwarding local memsets to loads.  For example, we optimize this:

short x(short *A) {
  memset(A, 1, sizeof(*A)*100);
  return A[42];
}

to 'return 257' instead of doing the load.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90695 91177308-0d34-0410-b5e6-96231b3b80d8