Add a useful overload of properlyDominates to PostDominanceInfo (swiftlang#34666)

Author: meg-gupta

include/swift/SIL/Dominance.h

@@ -149,6 +149,7 @@ class PostDominanceInfo : public PostDominatorTreeBase {
   PostDominanceInfo(SILFunction *F);
 
   bool properlyDominates(SILInstruction *A, SILInstruction *B);
+  bool properlyDominates(SILValue A, SILInstruction *B);
 
   void verify() const;
 

lib/SIL/Utils/Dominance.cpp

@@ -114,6 +114,16 @@ properlyDominates(SILInstruction *I1, SILInstruction *I2) {
   return true;
 }
 
+bool PostDominanceInfo::properlyDominates(SILValue A, SILInstruction *B) {
+  if (auto *Inst = A->getDefiningInstruction()) {
+    return properlyDominates(Inst, B);
+  }
+  if (auto *Arg = dyn_cast<SILArgument>(A)) {
+    return dominates(Arg->getParent(), B->getParent());
+  }
+  return false;
+}
+
 void PostDominanceInfo::verify() const {
   // Recompute.
   //