intel/brw: Add a idom_tree::dominates(a, b) helper.

Simpler to use than the existing methods.

Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29624>

src/intel/compiler/brw_cfg.h

@@ -511,6 +511,21 @@ namespace brw {
       bblock_t *
       intersect(bblock_t *b1, bblock_t *b2) const;
 
+      /**
+       * Returns true if block `a` dominates block `b`.
+       */
+      bool
+      dominates(const bblock_t *a, const bblock_t *b) const
+      {
+         while (a != b) {
+            if (b->num == 0)
+               return false;
+
+            b = parent(b);
+         }
+         return true;
+      }
+
       void
       dump() const;