Don't use a potentially expensive shift if all we want is one set bit.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186095 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/ConstantRange.cpp b/lib/Support/ConstantRange.cpp
index d770381..bb38cd1 100644
--- a/lib/Support/ConstantRange.cpp
+++ b/lib/Support/ConstantRange.cpp
@@ -433,7 +433,7 @@
     APInt LowerExt(DstTySize, 0);
     if (!Upper) // special case: [X, 0) -- not really wrapping around
       LowerExt = Lower.zext(DstTySize);
-    return ConstantRange(LowerExt, APInt(DstTySize, 1).shl(SrcTySize));
+    return ConstantRange(LowerExt, APInt::getOneBitSet(DstTySize, SrcTySize));
   }
 
   return ConstantRange(Lower.zext(DstTySize), Upper.zext(DstTySize));