glsl: refactor code to avoid static analyzer noise

glsl: refactor code to avoid static analyzer noise

Clang analyzer thinks struct_base_offset can be used uninitialized
because it doesn't know that glsl_type_is_struct_or_ifc returns
the same value for the same type.

Refactor the code to make it clear what is going on. As a side effect
this should be faster because glsl_get_length and
glsl_type_is_struct_or_ifc will be called only once (they are not
inline functions).

This is an alternative approach to
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12399.

Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12403>