Revert r356304: remove subreg parameter from MachineIRBuilder::buildCopy()
After review comments, it was preferred to not teach MachineIRBuilder about
non-generic instructions beyond using buildInstr().
For AArch64 I've changed the buildCopy() calls to buildInstr() + a
separate addReg() call.
This also relaxes the MachineIRBuilder's COPY checking more because it may
not always have a SrcOp given to it.