CodeGen: Create a proper ModuleSlotTracker for MachineInstr

Another follow-up related to r240848: try a little harder to share slot
tracking calculations within a single `MachineInstr` dump.  This is
unrelated to `MachineFunction::print()`, since that should be passing
through the function's `ModuleSlotTracker` by now, but could affect the
speed of dumping from a debugger if there is more than one IR-level
operand.

llvm-svn: 240852