nir/validate: Size the set of blocks to avoid rehashing.

We can use num_blocks (if it's been initialized by some pass indexing
blocks) to pre-size our table, which helps on validating shaders with many
blocks which would otherwise reallocate the set several times.

No statistically significant performance difference on softpipe
KHR-GL33.texture_swizzle.functional runtime (n=15).  A previous, similar
variant of this patch cut .3% of instructions in softpipe shader-db ./run
shaders/closed/steam/borderlands-2/35* (an arbitrary set of shaders that
completed in reasonable amount of time) according to callgrind.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7244>

src/compiler/nir/nir.c

@@ -371,6 +371,7 @@ nir_function_impl_create_bare(nir_shader *shader)
    exec_list_make_empty(&impl->locals);
    impl->reg_alloc = 0;
    impl->ssa_alloc = 0;
+   impl->num_blocks = 0;
    impl->valid_metadata = nir_metadata_none;
    impl->structured = true;
 

src/compiler/nir/nir_validate.c

@@ -1517,6 +1517,7 @@ validate_function_impl(nir_function_impl *impl, validate_state *state)
                                     sizeof(BITSET_WORD));
 
    _mesa_set_clear(state->blocks, NULL);
+   _mesa_set_resize(state->blocks, impl->num_blocks);
    collect_blocks(&impl->body, state);
    _mesa_set_add(state->blocks, impl->end_block);
    validate_assert(state, !exec_list_is_empty(&impl->body));