glsl: Remove bitfield_extract and bitfield_insert lowering

glsl: Remove bitfield_extract and bitfield_insert lowering

As far as I can tell, every driver that supports GLSL 1.30 or
GL_EXT_gpu_shader4 (and therefore also enables support for
GL_MESA_shader_integer_functions) also sets some subset of the various
NIR lower_bitfield_extract and lower_bitfield_insert flags.

v2: Declaration of 'result' still needs to be added to the IR. Noticed
by marge.

v3: Fix 'git rebase --autosquash' putting the v2 fix in the wrong
place. I've never seen that happen before. :(

Reviewed-by: Emma Anholt <emma@anholt.net> [v1]
Reviewed-by: Matt Turner <mattst88@gmail.com> [v1]
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20323>