[GlobalISel] Allow MachineIRBuilder to build subregister copies.

This relaxes some asserts about sizes, and adds an optional subreg parameter
to buildCopy().

Also update AArch64 instruction selector to use this in places where we
previously used MachineInstrBuilder manually.

Differential Revision: https://reviews.llvm.org/D59434

llvm-svn: 356304