nir/lower_locals_to_regs: Add bool bitsize knob

GLSL booleans (and hence bool derefs) may be translated either as 1-bit or
32-bit NIR registers, depending whether the backend uses nir_lower_bool_to_int32
or not. Add a knob for this and choose the right type for different backends.

Fixes nir_validate failure on
dEQP-VK.subgroups.ballot_broadcast.graphics.subgroupbroadcast_bvec3 run under
lavapipe. That test indexes into a bvec3 array, and gallivm first lowers bools
and then lowers derefs to registers, resulting in random 1-bit booleans mixed in
with 32-bit bools.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23804>

src/intel/compiler/brw_nir.c

@@ -1701,7 +1701,7 @@ brw_postprocess_nir(nir_shader *nir, const struct brw_compiler *compiler,
    OPT(nir_copy_prop);
    OPT(nir_opt_dce);
 
-   OPT(nir_lower_locals_to_regs);
+   OPT(nir_lower_locals_to_regs, 32);
 
    if (unlikely(debug_enabled)) {
       /* Re-index SSA defs so we print more sensible numbers. */