[RFC][GISel] Add a way to ignore COPY instructions in InstructionSelector
RFC to add a way to ignore COPY instructions when pattern-matching MIR in GISel.
- Add a new "GISelFlags" class to TableGen. Both `Pattern` and `PatFrags` defs can use it to alter matching behaviour.
- Flags start at zero and are scoped: the setter returns a `SaveAndRestore` object so that when the current scope ends, the flags are restored to their previous values. This allows child patterns to modify the flags without affecting the parent pattern.
- Child patterns always reuse the parent's pattern, but they can override its values. For more examples, see `GlobalISelEmitterFlags.td` tests.
- [AMDGPU] Use the IgnoreCopies flag in BFI patterns, which are known to be bothered by cross-regbank copies.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D136234