[CodeGenPrepare] Use const reference to avoid unnecessary APInt copy. NFC

[craig.topper]

Summary

Spotted while looking at Matthias' patches.

[RKSimon]

LGTM

llvm/lib/CodeGen/CodeGenPrepare.cpp

 //===- CodeGenPrepare.cpp - Prepare a function for code generation --------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
if (Arg->hasZExtAttr())
       ExtType = Instruction::ZExt;
   }
 
   auto *ExtInst = CastInst::Create(ExtType, Cond, NewType);
   ExtInst->insertBefore(SI);
   ExtInst->setDebugLoc(SI->getDebugLoc());
   SI->setCondition(ExtInst);
   for (auto Case : SI->cases()) {
-    APInt NarrowConst = Case.getCaseValue()->getValue();
+    const APInt &NarrowConst = Case.getCaseValue()->getValue();
     APInt WideConst = (ExtType == Instruction::ZExt) ?
                       NarrowConst.zext(RegWidth) : NarrowConst.sext(RegWidth);
     Case.setValue(ConstantInt::get(Context, WideConst));
   }
 
   return true;
 }