[CodeGen] Move printing MO_Immediate operands to MachineOperand::print

Work towards the unification of MIR and debug output by refactoring the
interfaces.

Add support for operand subreg index as an immediate to debug printing
and use ::print in the MIRPrinter.

Differential Review: https://reviews.llvm.org/D40965

llvm-svn: 320209