[AMDGPU] Fixes in ISelDAG path and GlobalISel path for 'bias' operand with A16 bit on

The LOD bias operand is of type 'half' when the A16-bit is ON' for MIMG instructions.
'bias' is only 16-bit but occupies 32-bits with upper 16-bits containing junk.
The patch fixes both the paths(ISelDAG and GlobalISel) for proper encoding of LOD bias operand.

Differential Revision: https://reviews.llvm.org/D111754