nir: add a bool bitsize lowering pass

nir: add a bool bitsize lowering pass

The pass lowers 1-bit booleans produced by NIR to the native bitsize
of the operations that produce them.

v2: change on lower_load_const_instr after upstream changes. Added
    TODO2 to explain it, as it was not properly tested yet (see
    already existing TODO) (Neil)

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3885>