[DAGCombiner] Avoid repeated calls to WideVT.getScalarSizeInBits() in DAGCombiner::mergeTruncStores. NFC (llvm#152231)

We already have a variable, WideNumBits, that contains the same
information. Use it and delay the creation of WideVT until we really
need it.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -9390,8 +9390,7 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
   LLVMContext &Context = *DAG.getContext();
   unsigned NumStores = Stores.size();
   unsigned WideNumBits = NumStores * NarrowNumBits;
-  EVT WideVT = EVT::getIntegerVT(Context, WideNumBits);
-  if (WideVT != MVT::i16 && WideVT != MVT::i32 && WideVT != MVT::i64)
+  if (WideNumBits != 16 && WideNumBits != 32 && WideNumBits != 64)
     return SDValue();
 
   // Check if all bytes of the source value that we are looking at are stored
@@ -9445,7 +9444,7 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
         SourceValue = WideVal;
 
       // Give up if the source value type is smaller than the store size.
-      if (SourceValue.getScalarValueSizeInBits() < WideVT.getScalarSizeInBits())
+      if (SourceValue.getScalarValueSizeInBits() < WideNumBits)
         return SDValue();
     }
 
@@ -9469,6 +9468,8 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
     OffsetMap[Offset] = ByteOffsetFromBase;
   }
 
+  EVT WideVT = EVT::getIntegerVT(Context, WideNumBits);
+
   assert(FirstOffset != INT64_MAX && "First byte offset must be set");
   assert(FirstStore && "First store must be set");