nir: Fix nir_validate fail after nir_lower_tex

nir: Fix nir_validate fail after nir_lower_tex

It is UB to initialize unions on the stack and rely on bits not covered
by the initialized union member to be zero.  Lets just simplify it and
move the entire nir_const_value off the stack.

While we're in there, sprinkle around some const.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3778
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7579>