[X86] combineGatherScatter - pull out repeated index sign bits code. NFC. (#132399)

Minor cleanup to make it easier to handle more index simplification and truncation to vXi32 types in future patches.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -56427,34 +56427,34 @@ static SDValue combineGatherScatter(SDNode *N, SelectionDAG &DAG,
   if (DCI.isBeforeLegalize()) {
     unsigned IndexWidth = Index.getScalarValueSizeInBits();
 
-    // Shrink constant indices if they are larger than 32-bits.
+    // Shrink indices if they are larger than 32-bits.
     // Only do this before legalize types since v2i64 could become v2i32.
     // FIXME: We could check that the type is legal if we're after legalize
     // types, but then we would need to construct test cases where that happens.
-    // FIXME: We could support more than just constant vectors, but we need to
-    // careful with costing. A truncate that can be optimized out would be fine.
-    // Otherwise we might only want to create a truncate if it avoids a split.
-    if (auto *BV = dyn_cast<BuildVectorSDNode>(Index)) {
-      if (BV->isConstant() && IndexWidth > 32 &&
-          DAG.ComputeNumSignBits(Index) > (IndexWidth - 32)) {
-        EVT NewVT = IndexVT.changeVectorElementType(MVT::i32);
+    if (IndexWidth > 32 && DAG.ComputeNumSignBits(Index) > (IndexWidth - 32)) {
+      EVT NewVT = IndexVT.changeVectorElementType(MVT::i32);
+
+      // FIXME: We could support more than just constant vectors, but we need to
+      // careful with costing. A truncate that can be optimized out would be
+      // fine. Otherwise we might only want to create a truncate if it avoids a
+      // split.
+      if (auto *BV = dyn_cast<BuildVectorSDNode>(Index)) {
+        if (BV->isConstant()) {
+          Index = DAG.getNode(ISD::TRUNCATE, DL, NewVT, Index);
+          return rebuildGatherScatter(GorS, Index, Base, Scale, DAG);
+        }
+      }
+
+      // Shrink any sign/zero extends from 32 or smaller to larger than 32 if
+      // there are sufficient sign bits. Only do this before legalize types to
+      // avoid creating illegal types in truncate.
+      if ((Index.getOpcode() == ISD::SIGN_EXTEND ||
+           Index.getOpcode() == ISD::ZERO_EXTEND) &&
+          Index.getOperand(0).getScalarValueSizeInBits() <= 32) {
         Index = DAG.getNode(ISD::TRUNCATE, DL, NewVT, Index);
         return rebuildGatherScatter(GorS, Index, Base, Scale, DAG);
       }
     }
-
-    // Shrink any sign/zero extends from 32 or smaller to larger than 32 if
-    // there are sufficient sign bits. Only do this before legalize types to
-    // avoid creating illegal types in truncate.
-    if ((Index.getOpcode() == ISD::SIGN_EXTEND ||
-         Index.getOpcode() == ISD::ZERO_EXTEND) &&
-        IndexWidth > 32 &&
-        Index.getOperand(0).getScalarValueSizeInBits() <= 32 &&
-        DAG.ComputeNumSignBits(Index) > (IndexWidth - 32)) {
-      EVT NewVT = IndexVT.changeVectorElementType(MVT::i32);
-      Index = DAG.getNode(ISD::TRUNCATE, DL, NewVT, Index);
-      return rebuildGatherScatter(GorS, Index, Base, Scale, DAG);
-    }
   }
 
   EVT PtrVT = TLI.getPointerTy(DAG.getDataLayout());