[SeparateConstOffsetFromGEP] Avoid use of ConstantExpr::getCast()

Try to constant fold, otherwise fall back to generating an
instruction.

Author: nikic

llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

@@ -661,15 +661,16 @@ Value *ConstantOffsetExtractor::applyExts(Value *V) {
   // in the reversed order.
   for (CastInst *I : llvm::reverse(ExtInsts)) {
     if (Constant *C = dyn_cast<Constant>(Current)) {
-      // If Current is a constant, apply s/zext using ConstantExpr::getCast.
-      // ConstantExpr::getCast emits a ConstantInt if C is a ConstantInt.
-      Current = ConstantExpr::getCast(I->getOpcode(), C, I->getType());
-    } else {
-      Instruction *Ext = I->clone();
-      Ext->setOperand(0, Current);
-      Ext->insertBefore(IP);
-      Current = Ext;
+      // Try to constant fold the cast.
+      Current = ConstantFoldCastOperand(I->getOpcode(), C, I->getType(), DL);
+      if (Current)
+        continue;
     }
+
+    Instruction *Ext = I->clone();
+    Ext->setOperand(0, Current);
+    Ext->insertBefore(IP);
+    Current = Ext;
   }
   return Current;
 }