[NFC][CodeGen] Minor cleanup in MachineInstr operand printing (#181892)
- Rename `getTiedOperandIdx` to `GetTiedOperandIdx` per LLVM CS.
- Do not compute tied operand index for defs, since tied operands are
printed only on uses.
- Restructure the `if` in the later operand printing loop to not compute
tied operand index/type for subreg-index imm operands.