[sil-cloner] Add a helper method for registering opened existentials remapping. NFC.

This is just a minor re-factoring of the existing code. It does not change the functionality.

Author: swiftix

include/swift/SIL/SILCloner.h

@@ -224,6 +224,11 @@ class SILCloner : protected SILVisitor<ImplClass> {
            "cloned instruction dropped debug scope");
   }
 
+  // Register a re-mapping for opened existentials.
+  void registerOpenedExistentialRemapping(ArchetypeType *From, CanType To) {
+    OpenedExistentialSubs[From] = To;
+  }
+
 protected:
 
   SILBuilder Builder;
@@ -1330,8 +1335,9 @@ SILCloner<ImplClass>::visitOpenExistentialAddrInst(OpenExistentialAddrInst *Inst
     = Inst->getType().getSwiftRValueType()->castTo<ArchetypeType>();
   assert(OpenedExistentialSubs.count(archetypeTy) == 0 && 
          "Already substituted opened existential archetype?");
-  OpenedExistentialSubs[archetypeTy] 
-    = ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType());
+  registerOpenedExistentialRemapping(
+      archetypeTy,
+      ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType()));
 
   getBuilder().setCurrentDebugScope(getOpScope(Inst->getDebugScope()));
   doPostProcess(Inst,
@@ -1352,8 +1358,9 @@ visitOpenExistentialMetatypeInst(OpenExistentialMetatypeInst *Inst) {
     openedType = cast<MetatypeType>(openedType).getInstanceType();
   }
   auto archetypeTy = cast<ArchetypeType>(openedType);
-  OpenedExistentialSubs[archetypeTy] 
-    = ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType());
+  registerOpenedExistentialRemapping(
+      archetypeTy,
+      ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType()));
 
   if (!Inst->getOperand()->getType().canUseExistentialRepresentation(
           Inst->getModule(), ExistentialRepresentation::Class)) {
@@ -1379,8 +1386,9 @@ visitOpenExistentialRefInst(OpenExistentialRefInst *Inst) {
   // Create a new archetype for this opened existential type.
   auto archetypeTy
     = Inst->getType().getSwiftRValueType()->castTo<ArchetypeType>();
-  OpenedExistentialSubs[archetypeTy] 
-    = ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType());
+  registerOpenedExistentialRemapping(
+      archetypeTy,
+      ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType()));
 
   getBuilder().setCurrentDebugScope(getOpScope(Inst->getDebugScope()));
   doPostProcess(Inst,
@@ -1396,8 +1404,9 @@ visitOpenExistentialBoxInst(OpenExistentialBoxInst *Inst) {
   // Create a new archetype for this opened existential type.
   auto archetypeTy
     = Inst->getType().getSwiftRValueType()->castTo<ArchetypeType>();
-  OpenedExistentialSubs[archetypeTy] 
-    = ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType());
+  registerOpenedExistentialRemapping(
+      archetypeTy,
+      ArchetypeType::getOpened(archetypeTy->getOpenedExistentialType()));
 
   getBuilder().setCurrentDebugScope(getOpScope(Inst->getDebugScope()));
   doPostProcess(Inst,