[TII] Use optional destination and source pair as a return value; NFC

[TII] Use optional destination and source pair as a return value; NFC

Refactor usage of isCopyInstrImpl, isCopyInstr and isAddImmediate methods
to return optional machine operand pair of destination and source
registers.

Patch by Nikola Prica

Differential Revision: https://reviews.llvm.org/D69622