[MachineVerifier][GlobalISel] NFC, Improving MO printing and refactoring visitMachineInstrBefore

This is an NFC pre-commit for the following "Checking that generic
instrs have LLTs on all vregs" commit.

This overloads MachineOperand::print to make it possible to print LLTs
with standalone machine operands.

This also overloads MachineVerifier::print(...MachineOperand...) with
an optional LLT using the newly introduced MachineOperand::print
variant; no actual calls added.

This also refactors MachineVerifier::visitMachineInstrBefore in the
parts dealing with all generic instructions (checking Selected
property, LLTs, and phys regs).

llvm-svn: 331693