glsl: Fix assert fails when assignment expressions are in array sizes.

glsl: Fix assert fails when assignment expressions are in array sizes.

Karol Herbst's fuzzing efforts discovered that we would hit the
following assert:

   assert(dummy_instructions.is_empty());

when processing an illegal array size expression of

   float[(1=1)?1:1] t;

In do_assignment, we realized we needed an rvalue for (1 = 1), and
generated a temporary variable and assignment from the RHS.  We've
already flagged an error (non-lvalue in assignment), and return a bogus
value as the rvalue.  But process_array_size sees the bogus value, which
happened to be a constant expression, and rightly assumes that
processing a constant expression shouldn't have generated any code.
instructions.

To handle this, make do_assignment not generate any temps or assignments
when it's already raised an error - just return an error value directly.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98694
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>