[Mem2Reg] Omit lexical moves for lexical values.

When promoting an `alloc_stack [lexical]` that is "owned" (i.e. not a
store_borrow location), omit lexical lifetimes (represented with
`move_value [lexical]` instructions) if the value being stored is
already lexical.  Such moves are redundant.

rdar://99160718

Author: nate-chandler

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -57,6 +57,7 @@ STATISTIC(NumInstRemoved,        "Number of Instructions removed");
 
 static bool lexicalLifetimeEnsured(AllocStackInst *asi);
 static bool isGuaranteedLexicalValue(SILValue src);
+static bool isOwnedLexicalValue(SILValue src);
 
 namespace {
 
@@ -96,9 +97,11 @@ class LiveValues {
       if (!lexicalLifetimeEnsured(asi)) {
         return stored;
       }
-      // We should have created a move of the @owned stored value.
-      assert(move);
-      return move;
+      auto storedIsLexical = stored && isOwnedLexicalValue(stored);
+      // If the value was already lexical, we use it directly.  Otherwise, a new
+      // move_value [lexical] is used.
+      assert(storedIsLexical || move);
+      return storedIsLexical ? stored : move;
     }
 
     bool canEndLexicalLifetime() {
@@ -107,7 +110,8 @@ class LiveValues {
       // to end a lexical lifetime. In that case, the lifetime end will be
       // added later, when we have enough information, namely the live in
       // values, to end it.
-      return move;
+      auto storedIsLexical = stored && isOwnedLexicalValue(stored);
+      return storedIsLexical ? stored : move;
     }
   };
   struct Guaranteed {
@@ -522,6 +526,10 @@ static bool lexicalLifetimeEnsured(AllocStackInst *asi) {
          !asi->getElementType().isTrivial(*asi->getFunction());
 }
 
+static bool isOwnedLexicalValue(SILValue src) {
+  return src->getOwnershipKind() == OwnershipKind::Owned && src->isLexical();
+}
+
 static bool isGuaranteedLexicalValue(SILValue src) {
   return src->getOwnershipKind() == OwnershipKind::Guaranteed &&
          src->isLexical();
@@ -542,6 +550,9 @@ beginOwnedLexicalLifetimeAfterStore(AllocStackInst *asi, StoreInst *inst) {
   SILValue stored = inst->getOperand(CopyLikeInstruction::Src);
   SILLocation loc = RegularLocation::getAutoGeneratedLocation(inst->getLoc());
 
+  if (isOwnedLexicalValue(stored)) {
+    return {LiveValues::forOwned(stored, {}), /*isStorageValid*/ true};
+  }
   MoveValueInst *mvi = nullptr;
   SILBuilderWithScope::insertAfter(inst, [&](SILBuilder &builder) {
     mvi = builder.createMoveValue(loc, stored, /*isLexical*/ true);
@@ -1063,6 +1074,10 @@ StackAllocationPromoter::getLiveOutValues(BlockSetVector &phiBlocks,
         auto values = LiveValues::forGuaranteed(stored, borrow);
         return values;
       }
+      if (isOwnedLexicalValue(stored)) {
+        auto values = LiveValues::forOwned(stored, {});
+        return values;
+      }
       auto move = cast<MoveValueInst>(inst->getNextInstruction());
       auto values = LiveValues::forOwned(stored, move);
       return values;