glsl: Hook up loop_variable_state destructor to plug a memory leak.

glsl: Hook up loop_variable_state destructor to plug a memory leak.

While ~loop_state() is already freeing the loop_variable_state objects
via ralloc_free(this->mem_ctx), the ~loop_variable_state() destructor
was never getting called, so the hash table inside loop_variable_state
was never getting destroyed.

Fixes a memory leak in any shader with loops.

NOTE: This is a candidate for stable release branches.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>