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/broadcom/compiler/vir.c

@@ -1752,6 +1752,7 @@ v3d_attempt_compile(struct v3d_compile *c)
         NIR_PASS(_, c->s, nir_lower_vars_to_scratch,
                  nir_var_function_temp,
                  0,
+                 glsl_get_natural_size_align_bytes,
                  glsl_get_natural_size_align_bytes);
 
         NIR_PASS(_, c->s, v3d_nir_lower_global_2x32);