[NVPTXAsmPrinter] Avoid use of ConstantExpr::getIntegerCast() (NFC)

Sync the implementation with the original AsmPrinter code.

Author: nikic

llvm/lib/Target/NVPTX/NVPTXAsmPrinter.cpp

@@ -2024,9 +2024,12 @@ NVPTXAsmPrinter::lowerConstantForGV(const Constant *CV, bool ProcessingGeneric)
     // Handle casts to pointers by changing them into casts to the appropriate
     // integer type.  This promotes constant folding and simplifies this code.
     Constant *Op = CE->getOperand(0);
-    Op = ConstantExpr::getIntegerCast(Op, DL.getIntPtrType(CV->getType()),
-                                      false/*ZExt*/);
-    return lowerConstantForGV(Op, ProcessingGeneric);
+    Op = ConstantFoldIntegerCast(Op, DL.getIntPtrType(CV->getType()),
+                                 /*IsSigned*/ false, DL);
+    if (Op)
+      return lowerConstantForGV(Op, ProcessingGeneric);
+
+    break; // Error
   }
 
   case Instruction::PtrToInt: {