Revert r356304: remove subreg parameter from MachineIRBuilder::buildCopy()

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356396 91177308-0d34-0410-b5e6-96231b3b80d8