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>
glsl/linker: Validate IR just before reparenting.

If reparent_ir() is called on invalid IR, then there's a danger that
it will fail to reparent all of the necessary nodes.  For example, if
the IR contains an ir_dereference_variable which refers to an
ir_variable that's not in the tree, that ir_variable won't get
reparented, resulting in subtle use-after-free bugs once the
non-reparented nodes are freed.  (This is exactly what happened in the
bug fixed by the previous commit).

This patch makes this kind of bug far easier to track down, by
transforming it from a use-after-free bug into an explicit IR
validation error.