Fix a regression in Intel Gfx6 and clean up some GLSL lowering

GLSL IR opcodes generated for bitfieldExtract and bitfieldInsert are lowered by lower_instructions. 4dff3ff0 ("nir/opt_algebraic: Optimize open coded bfm.") adds an optimization that can rematerialize nir_op_bfm that was prevented by the GLSL IR lowering.

It appears that every piece of hardware, except older Intel GPUS, that has real integers (i.e., lower_bitops is not set) also sets lower_bitfield_extract_to_shifts and lower_bitfield_insert_to_shifts. The first commit modifies the Intel compiler to do this for Gfx6 and older.

With that in place, several of the lowering operations performed by lower_instructions are unnecessary, so several other commits in the series remove them.