draw: Ensure that prepare is always run after LLVM garbagge collection.

Should avoid dangling pointer derreference with

  glean --run results --overwrite --quick --tests texSwizzle

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit 07635a4799)

src/gallium/auxiliary/draw/draw_llvm.c

@@ -65,8 +65,13 @@ static void
 draw_llvm_garbage_collect_callback(void *cb_data)
 {
    struct draw_llvm *llvm = (struct draw_llvm *) cb_data;
+   struct draw_context *draw = llvm->draw;
    struct draw_llvm_variant_list_item *li;
 
+   /* Ensure prepare will be run and shaders recompiled */
+   assert(!draw->suspend_flushing);
+   draw_do_flush(draw, DRAW_FLUSH_STATE_CHANGE);
+
    /* free all shader variants */
    li = first_elem(&llvm->vs_variants_list);
    while (!at_end(&llvm->vs_variants_list, li)) {