[AArch64][GlobalISel] Inline tiny memcpy et al at -O0.

FastISel already does this since the initial arm64 port was upstreamed, so
it seems there are no issues with doing this at -O0 for very small memcpys.

Gives a 0.2% geomean code size improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D65758

llvm-svn: 367919