[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

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