[GlobalISel] Port `SimplifyDemandedBits` over (#198808)
This patch port SelectionDAG-style demanded-bits simplification to
`CombinerHelper::simplifyDemandedBits`, initially handling scalar
`G_AND` and `G_OR`.
The implementation follows the SelectionDAG version. Specifically, it
recursively propagates demanded bits through operands, simplifying them
and computing `KnownBits` in the same walk. It preserves the bits
required by other users, handles zero-demand cases, and drops
poison-generating flags or invalidates affected debug values when
necessary.
The minimal consumer is the opt-in `simplify_demanded_bits` rule for
`G_AND` and `G_OR` in the AArch64 post-legalizer combiner. It also
includes unit tests, isolated-rule and full-combiner MIR tests, and the
resulting AArch64 codegen checks are included.
To keep this PR small, additional consumers and selection work remain on
separate branches for future PRs:
- [Constant-shift
handling](https://github.com/llvm/llvm-project/pull/230636)
- [Truncation-of-shift
integration](https://github.com/lialan/llvm-project/tree/users/lialan/gisel_trunc_shift_demanded_bits).
- [Narrowing truncations of constant right
shifts](https://github.com/lialan/llvm-project/tree/users/lialan/gisel_narrow_trunc_shr).
- [AArch64 G_UBFX
selection](https://github.com/lialan/llvm-project/tree/pr-stack/aarch64-ubfx).
Tracks #150515.
