Add checkReachingBlockDominates utility.

This is a tiny routine that is useful for writing assertions to be
used for quick sanity checks without computing Dominance.

It doesn't always make sense to require a pass to maintain and pass
around Dominance just for an assert.

include/swift/SIL/BasicBlockUtils.h

@@ -148,6 +148,10 @@ void findJointPostDominatingSet(
     function_ref<void(SILBasicBlock *)> foundJointPostDomSetCompletionBlocks,
     function_ref<void(SILBasicBlock *)> inputBlocksInJointPostDomSet = {});
 
+#ifndef NDEBUG
+bool checkDominates(SILBasicBlock *sourceBlock, SILBasicBlock *destBlock);
+#endif
+
 } // namespace swift
 
 #endif