glsl: Make constant propagation's folder not propagate into an LHS.

glsl: Make constant propagation's folder not propagate into an LHS.

opt_constant_propagation.cpp contains constant folding code which can
actually do constant propagation in some cases.  It was happily
propagating constants into the left-hand-side of assignments.

For example,

   (assign () (var_ref temp) (constant ...))

would brilliantly be turned into:

   (assign () (constant ...) (constant ....))

This is a bigger hammer than necessary - it prevents propagation
into the left-hand-side altogether.  We could certainly do better
someday.  Notably, the constant propagation pass itself already
takes this approach - it's just the constant propagation pass's
built-in constant folding code (which actually propagates, too)
that was broken.

No change in shader-db, but prevents regressions after future commits.
It seems plausible that this could be hit today, but I haven't seen it
happen.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>