Extend the CoalescerPair interface to handle symmetric sub-register copies.

Extend the CoalescerPair interface to handle symmetric sub-register copies.

Now both SrcReg and DstReg can be sub-registers of the final coalesced
register.

CoalescerPair::setRegisters still rejects such copies because
RegisterCoalescer doesn't yet handle them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156848 91177308-0d34-0410-b5e6-96231b3b80d8