[GlobalISel][NFC] Add MachineInstr::getFirst[N]{Regs,LLTs}() helpers to extract regs & types.

These reduce the typing and clutter from:
Register Dst = MI.getOperand(0).getReg();
Register Src1 = MI.getOperand(1).getReg();
Register Src2 = MI.getOperand(2).getReg();
Register Src3 = MI.getOperand(3).getReg();
LLT DstTy = MRI.getType(Dst);
... etc etc

To just:
auto [Dst, Src1, Src2, Src3] = MI.getFirst4Regs();
auto [DstTy, Src1Ty, Src2Ty, Src3Ty] = MI.getFirst4LLTs();

Or even more concise:
auto [Dst, DstTy, Src1, Src1Ty, Src2, Src2Ty, Src3, Src3Ty] =
     MI.getFirst4RegLLTs();

Differential Revision: https://reviews.llvm.org/D144687