[NFC] IRGen: Clarify metadata collection helper.

Gather the circumstances under which the collector should be used and
what it should collect separately and then collect only in one place if
collection should happen.

Author: nate-chandler

lib/IRGen/Outlining.cpp

@@ -151,22 +151,29 @@ irgen::getTypeAndGenericSignatureForManglingOutlineFunction(SILType type) {
 }
 
 bool TypeInfo::withWitnessableMetadataCollector(
-    IRGenFunction &IGF, SILType T, LayoutIsNeeded_t needsLayout,
+    IRGenFunction &IGF, SILType T, LayoutIsNeeded_t mayNeedLayout,
     DeinitIsNeeded_t needsDeinit,
     llvm::function_ref<void(OutliningMetadataCollector &)> invocation) const {
+  bool needsCollector = false;
+  LayoutIsNeeded_t needsLayout = LayoutIsNotNeeded;
   if (!T.hasLocalArchetype() &&
       !IGF.outliningCanCallValueWitnesses()) {
-    OutliningMetadataCollector collector(IGF, needsLayout, needsDeinit);
+    needsCollector = true;
     if (T.hasArchetype()) {
-      collectMetadataForOutlining(collector, T);
+      needsLayout = LayoutIsNeeded;
     }
-    invocation(collector);
-    return true;
+  } else if (!T.hasArchetype()) {
+    needsCollector = true;
+    // The implementation will call vwt in this case.
+    needsLayout = LayoutIsNotNeeded;
   }
 
-  if (!T.hasArchetype()) {
-    // The implementation will call vwt in this case.
+  if (needsCollector) {
     OutliningMetadataCollector collector(IGF, needsLayout, needsDeinit);
+    if (needsDeinit || needsLayout) {
+      // Only collect if anything would be collected.
+      collectMetadataForOutlining(collector, T);
+    }
     invocation(collector);
     return true;
   }
@@ -198,10 +205,8 @@ void TypeInfo::callOutlinedCopy(IRGenFunction &IGF, Address dest, Address src,
 }
 
 void OutliningMetadataCollector::emitCallToOutlinedCopy(
-                            Address dest, Address src,
-                            SILType T, const TypeInfo &ti, 
-                            IsInitialization_t isInit, IsTake_t isTake) const {
-  assert(needsLayout);
+    Address dest, Address src, SILType T, const TypeInfo &ti,
+    IsInitialization_t isInit, IsTake_t isTake) const {
   assert(!needsDeinit);
   llvm::SmallVector<llvm::Value *, 4> args;
   args.push_back(IGF.Builder.CreateElementBitCast(src, ti.getStorageType())
@@ -387,8 +392,7 @@ void TypeInfo::callOutlinedDestroy(IRGenFunction &IGF,
 }
 
 void OutliningMetadataCollector::emitCallToOutlinedDestroy(
-                      Address addr, SILType T, const TypeInfo &ti) const {
-  assert(needsLayout);
+    Address addr, SILType T, const TypeInfo &ti) const {
   assert(needsDeinit);
   llvm::SmallVector<llvm::Value *, 4> args;
   args.push_back(IGF.Builder.CreateElementBitCast(addr, ti.getStorageType())