glsl_to_nir: Use a variable's constant_value if it wasn't const-propped out.

glsl_to_nir: Use a variable's constant_value if it wasn't const-propped out.

glsl has been constant-propagating out references to ir->constant_value
(the value of a variable declared as const), but we can get rid of that
whole pass if we just have glsl-to-nir hand the constant propagating
problem off to NIR.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21751>