glsl/ast: update rhs in addition to the var's constant_value

glsl/ast: update rhs in addition to the var's constant_value

We continue in the code to do some more things with the rhs, including
setting a constant initializer. If the type is wrong, this causes some
confusion down the line, leading to assertions. This makes sure that the
rhs processing continues to flow as-if the type was correct to start
with (even though the state has been marked as an error state).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101766
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org