APFloat: Use isDenormal instead of hand-rolled code to check for denormals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183072 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index 4efbaf7..16586fb 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -3683,7 +3683,7 @@
 
   // Avoid multiplication with a denormal, it is not safe on all platforms and
   // may be slower than a normal division.
-  if (reciprocal.significandMSB() + 1 < reciprocal.semantics->precision)
+  if (reciprocal.isDenormal())
     return false;
 
   assert(reciprocal.category == fcNormal &&