X86 memcpy: use REPMOVSB instead of REPMOVS{Q,D,W} for inline copies

when the subtarget has fast strings.

This has two advantages:
  - Speed is improved. For example, on Haswell thoughput improvements increase
    linearly with size from 256 to 512 bytes, after which they plateau:
    (e.g. 1% for 260 bytes, 25% for 400 bytes, 40% for 508 bytes).
  - Code is much smaller (no need to handle boundaries).

llvm-svn: 300957