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.

llvm-svn: 356396