[PGO] Memory intrinsic calls optimization based on profiled size

This patch optimizes two memory intrinsic operations: memset and memcpy based
on the profiled size of the operation. The high level transformation is like:
  mem_op(..., size)
  ==>
  switch (size) {
    case s1:
       mem_op(..., s1);
       goto merge_bb;
    case s2:
       mem_op(..., s2);
       goto merge_bb;
    ...
    default:
       mem_op(..., size);
       goto merge_bb;
    }
  merge_bb:

Differential Revision: http://reviews.llvm.org/D28966

llvm-svn: 299446