[AMDGPU] simplifyI24 - replace GetDemandedBits with SimplifyMultipleUseDemandedBits

GetDemandedBits mostly just calls SimplifyMultipleUseDemandedBits now, but it does a very blunt constant simplification that SimplifyMultipleUseDemandedBits avoids.

If we need to demand bits from constants we should handle this through ShrinkDemandedConstant/targetShrinkDemandedConstant.

@arsenm confirmed that the sign extended immediates are better for code size.

Differential Revision: https://reviews.llvm.org/D74857