nir/lower_vars_to_scratch: calculate threshold-limited variable size separately

ir3's lowering of variables to scratch memory has to treat 8-bit values as
16-bit ones when comparing such value's size against the given threshold
since those values are handled through 16-bit half-registers. But those
values can still use natural 8-bit size and alignment for storing inside
scratch memory.

nir_lower_vars_to_scratch now accepts two size-and-alignment functions,
one used for calculating the variable size and the other for calculating
the size and alignment needed for storing inside scratch memory. Non-ir3
uses of this pass can just duplicate the currently-used function. ir3
provides a separate variable-size function that special-cases 8-bit types.

Signed-off-by: Zan Dobersek <zdobersek@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29875>

src/asahi/compiler/agx_compile.c

@@ -3293,7 +3293,7 @@ agx_preprocess_nir(nir_shader *nir, const nir_shader *libagx)
 
    /* Lower large arrays to scratch and small arrays to csel */
    NIR_PASS(_, nir, nir_lower_vars_to_scratch, nir_var_function_temp, 16,
-            glsl_get_natural_size_align_bytes);
+            glsl_get_natural_size_align_bytes, glsl_get_natural_size_align_bytes);
    NIR_PASS(_, nir, nir_lower_indirect_derefs, nir_var_function_temp, ~0);
    NIR_PASS(_, nir, nir_split_var_copies);
    NIR_PASS(_, nir, nir_lower_global_vars_to_local);