[GlobalISel] Fix legality checks for G_UBFX combines

1. Fix CombinerHelper::matchBitfieldExtractFromAnd to check legality
   with the correct types for the G_UBFX that it builds.
2. Fix AMDGPUTargetLowering::isConstantUnsignedBitfieldExtractLegal to
   match the legality rules: result and first operand can be s32 or s64
   but the "shift amount" operands are always s32.
3. Add AMDGPU tests where the post-legalizer combiner would create
   illegal MIR without the above fixes.

Differential Revision: https://reviews.llvm.org/D116802