glsl: Fix lowering of direct assignment in lower_clip_distance.

glsl: Fix lowering of direct assignment in lower_clip_distance.

In commit 065da16 (glsl: Convert lower_clip_distance_visitor to be an
ir_rvalue_visitor), we failed to notice that since
lower_clip_distance_visitor overrides visit_leave(ir_assignment *),
ir_rvalue_visitor::visit_leave(ir_assignment *) wasn't getting called.
As a result, clip distance dereferences appearing directly on the
right hand side of an assignment (not in a subexpression) weren't
getting properly lowered.  This caused an ir_dereference_variable node
to be left in the IR that referred to the old gl_ClipDistance
variable.  However, since the lowering pass replaces gl_ClipDistance
with gl_ClipDistanceMESA, this turned into a dangling pointer when the
IR got reparented.

Prior to the introduction of geometry shaders, this bug was unlikely
to arise, because (a) reading from gl_ClipDistance[i] in the fragment
shader was rare, and (b) when it happened, it was likely that it would
either appear in a subexpression, or be hoisted into a subexpression
by tree grafting.

However, in a geometry shader, we're likely to see a statement like
this, which would trigger the bug:

    gl_ClipDistance[i] = gl_in[j].gl_ClipDistance[i];

This patch causes
lower_clip_distance_visitor::visit_leave(ir_assignment *) to call the
base class visitor, so that the right hand side of the assignment is
properly lowered.

Fixes piglit test:
- spec/glsl-1.50/execution/geometry/clip-distance-itemized-copy

Cc: Ian Romanick <idr@freedesktop.org>
Cc: "9.2" <mesa-stable@lists.freedesktop.org>
Cc: "10.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>