[AIX] Lower some memory intrinsics to millicode functions on AIX

Summary: Currently we lower MEMCPY/MEMMOVE/MEMSET/BZERO to the corresponding libc functions. And the libc functions call the millicode functions on AIX. We can lower these intrinsics directly to save one call layer.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D143997