glsl: Make ir_assignment::condition private

And add get_condition().

This proof that nothing remains that could possibly set ::condition to
anything other than NULL.

v2: Fix bad rebase.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14573>

src/compiler/glsl/ir_clone.cpp

@@ -259,7 +259,7 @@ ir_texture::clone(void *mem_ctx, struct hash_table *ht) const
 ir_assignment *
 ir_assignment::clone(void *mem_ctx, struct hash_table *ht) const
 {
-   assert(this->condition == NULL);
+   assert(this->get_condition() == NULL);
 
    return new(mem_ctx) ir_assignment(this->lhs->clone(mem_ctx, ht),
                                      this->rhs->clone(mem_ctx, ht),