Teach ConvertUsesToScalar to handle memset, allowing it to handle

Teach ConvertUsesToScalar to handle memset, allowing it to handle
crazy cases like:

struct f {  int A, B, C, D, E, F; };
short test4() {
  struct f A;
  A.A = 1;
  memset(&A.B, 2, 12);
  return A.C;
}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63596 91177308-0d34-0410-b5e6-96231b3b80d8