[DAGCombiner] clean up getStoreMergeCandidates(); NFC

1. Move bailouts and local var declarations.
2. Convert if-chain to switch on StoreSource with unreachable default.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -16100,23 +16100,19 @@ void DAGCombiner::getStoreMergeCandidates(
     StoreSDNode *St, SmallVectorImpl<MemOpLink> &StoreNodes,
     SDNode *&RootNode) {
   // This holds the base pointer, index, and the offset in bytes from the base
-  // pointer.
+  // pointer. We must have a base and an offset. Do not handle stores to undef
+  // base pointers.
   BaseIndexOffset BasePtr = BaseIndexOffset::match(St, DAG);
-  EVT MemVT = St->getMemoryVT();
-
-  SDValue Val = peekThroughBitcasts(St->getValue());
-  // We must have a base and an offset.
-  if (!BasePtr.getBase().getNode())
-    return;
-
-  // Do not handle stores to undef base pointers.
-  if (BasePtr.getBase().isUndef())
+  if (!BasePtr.getBase().getNode() || BasePtr.getBase().isUndef())
     return;
 
+  SDValue Val = peekThroughBitcasts(St->getValue());
   StoreSource StoreSrc = getStoreSource(Val);
   assert(StoreSrc != StoreSource::Unknown && "Expected known source for store");
-  BaseIndexOffset LBasePtr;
+
   // Match on loadbaseptr if relevant.
+  EVT MemVT = St->getMemoryVT();
+  BaseIndexOffset LBasePtr;
   EVT LoadVT;
   if (StoreSrc == StoreSource::Load) {
     auto *Ld = cast<LoadSDNode>(Val);
@@ -16146,37 +16142,38 @@ void DAGCombiner::getStoreMergeCandidates(
     // Allow merging constants of different types as integers.
     bool NoTypeMatch = (MemVT.isInteger()) ? !MemVT.bitsEq(Other->getMemoryVT())
                                            : Other->getMemoryVT() != MemVT;
-    if (StoreSrc == StoreSource::Load) {
+    switch (StoreSrc) {
+    case StoreSource::Load: {
       if (NoTypeMatch)
         return false;
-      // The Load's Base Ptr must also match
-      if (LoadSDNode *OtherLd = dyn_cast<LoadSDNode>(OtherBC)) {
-        BaseIndexOffset LPtr = BaseIndexOffset::match(OtherLd, DAG);
-        if (LoadVT != OtherLd->getMemoryVT())
-          return false;
-        // Loads must only have one use.
-        if (!OtherLd->hasNUsesOfValue(1, 0))
-          return false;
-        // The memory operands must not be volatile/indexed/atomic.
-        // TODO: May be able to relax for unordered atomics (see D66309)
-        if (!OtherLd->isSimple() ||
-            OtherLd->isIndexed())
-          return false;
-        // Don't mix temporal loads with non-temporal loads.
-        if (cast<LoadSDNode>(Val)->isNonTemporal() != OtherLd->isNonTemporal())
-          return false;
-        if (!(LBasePtr.equalBaseIndex(LPtr, DAG)))
-          return false;
-      } else
+      // The Load's Base Ptr must also match.
+      auto *OtherLd = dyn_cast<LoadSDNode>(OtherBC);
+      if (!OtherLd)
+        return false;
+      BaseIndexOffset LPtr = BaseIndexOffset::match(OtherLd, DAG);
+      if (LoadVT != OtherLd->getMemoryVT())
+        return false;
+      // Loads must only have one use.
+      if (!OtherLd->hasNUsesOfValue(1, 0))
+        return false;
+      // The memory operands must not be volatile/indexed/atomic.
+      // TODO: May be able to relax for unordered atomics (see D66309)
+      if (!OtherLd->isSimple() || OtherLd->isIndexed())
+        return false;
+      // Don't mix temporal loads with non-temporal loads.
+      if (cast<LoadSDNode>(Val)->isNonTemporal() != OtherLd->isNonTemporal())
         return false;
+      if (!(LBasePtr.equalBaseIndex(LPtr, DAG)))
+        return false;
+      break;
     }
-    if (StoreSrc == StoreSource::Constant) {
+    case StoreSource::Constant:
       if (NoTypeMatch)
         return false;
       if (!(isa<ConstantSDNode>(OtherBC) || isa<ConstantFPSDNode>(OtherBC)))
         return false;
-    }
-    if (StoreSrc == StoreSource::Extract) {
+      break;
+    case StoreSource::Extract:
       // Do not merge truncated stores here.
       if (Other->isTruncatingStore())
         return false;
@@ -16185,6 +16182,9 @@ void DAGCombiner::getStoreMergeCandidates(
       if (OtherBC.getOpcode() != ISD::EXTRACT_VECTOR_ELT &&
           OtherBC.getOpcode() != ISD::EXTRACT_SUBVECTOR)
         return false;
+      break;
+    default:
+      llvm_unreachable("Unhandled store source for merging");
     }
     Ptr = BaseIndexOffset::match(Other, DAG);
     return (BasePtr.equalBaseIndex(Ptr, DAG, Offset));