[TableGen][GlobalISel] Add MIR Pattern Builtins

Adds a new feature to MIR patterns: builtin instructions.
They offer some additional capabilities that currently cannot be expressed without falling back to C++ code.
There are two builtins added with this patch, but more can be added later as new needs arise:
 - GIReplaceReg
 - GIEraseRoot

Depends on D158714, D158713

Reviewed By: arsenm, aemerson

Differential Revision: https://reviews.llvm.org/D158975