GlobalISel: Introduce GenericMachineInstr classes and derivatives for idiomatic LLVM RTTI.

This adds some level of type safety, allows helper functions to be added for
specific opcodes for free, and also allows us to succinctly check for class
membership with the usual dyn_cast/isa/cast functions.

To start off with, add variants for the different load/store operations with some
places using it.

Differential Revision: https://reviews.llvm.org/D105751