glsl: Avoid excess tree walking when folding ir_dereference_arrays.

glsl: Avoid excess tree walking when folding ir_dereference_arrays.

If an ir_dereference_array has non-constant components, there's no
point in trying to evaluate its value (which involves walking down
the tree and possibly allocating memory for portions of the subtree
which are constant).

This also removes convoluted tree walking in opt_constant_folding(),
which tries to fold constants while walking up the tree.  No need to
walk down, then up, then down again.

We did this for swizzles and expressions already, but I was lazy
back in the day and didn't do this for ir_dereference_array.

No change in shader-db.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>