[MemCpyOpt] Optimize double-storing by memset+memcpy.

[MemCpyOpt] Optimize double-storing by memset+memcpy.

A common idiom in some code is to do the following:

  memset(dst, 0, dst_size);
  memcpy(dst, src, src_size);

Some of the memset is redundant; instead, we can do:

  memcpy(dst, src, src_size);
  memset(dst + src_size, 0,
         dst_size <= src_size ? 0 : dst_size - src_size);

Original patch by: Joel Jones
Differential Revision: http://reviews.llvm.org/D498

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235232 91177308-0d34-0410-b5e6-96231b3b80d8