nir/lower_subgroups: Lower ballot intrinsics to the specified bit size

Ballot intrinsics return a bitfield of subgroups.  In GLSL and some
SPIR-V extensions, they return a uint64_t.  In SPV_KHR_shader_ballot,
they return a uvec4.  Also, some back-ends would rather pass around
32-bit values because it's easier than messing with 64-bit all the time.
To solve this mess, we make nir_lower_subgroups take a new parameter
called ballot_bit_size and it lowers whichever thing it gets in from the
source language (uint64_t or uvec4) to a scalar with the specified
number of bits.  This replaces a chunk of the old lowering code.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/intel/compiler/brw_compiler.c

@@ -57,7 +57,6 @@ static const struct nir_shader_compiler_options scalar_nir_options = {
    .lower_unpack_snorm_4x8 = true,
    .lower_unpack_unorm_2x16 = true,
    .lower_unpack_unorm_4x8 = true,
-   .max_subgroup_size = 32,
    .max_unroll_iterations = 32,
 };