glsl: Remove bonus tree walking in opt_constant_folding().

glsl: Remove bonus tree walking in opt_constant_folding().

It looks like this was missed when converting opt_constant_folding()
from a hierarchical visitor to an rvalue visitor in 6606fde3.

ir_rvalue_visitor already processes values on the way back up the tree,
so we will have already visited every child node.  There's no point in
doing it again.

No change in shader-db.

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