tgsi: align the definition of BFI & [UI]BFE with GLSL

tgsi: align the definition of BFI & [UI]BFE with GLSL

As previously written, these opcodes use the SM5 semantics which is
incompatible with GLSL when bits == 0, offset == 32.

At some point we may want to add BFI_SM5 etc. opcodes, but all users
currently either want (and expect!) the GLSL semantics or don't care.

Bitfield inserts are generated by the GLSL lower_instructions and
lower_packing_builtins passes with constant bits and offset arguments,
so any workaround code that drivers may have to emit to follow GLSL
semantics should be optimized away easily for those uses.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>