glsl/linker: Validate IR just before reparenting.

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.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>