[AMDGPU][GlobalISel] Compute known bits for zero-extending loads

Implement computeKnownBitsForTargetInstr for G_AMDGPU_BUFFER_LOAD_UBYTE
and G_AMDGPU_BUFFER_LOAD_USHORT. This allows generic combines to remove
some unnecessary G_ANDs.

Differential Revision: https://reviews.llvm.org/D89316