APFloat::roundToIntegral: Special values don't keep the exponent value up to date, don't rely on it.

Add a couple of unit tests for special floats. Fixes 13929, found by MemorySanitizer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164698 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/APFloat.cpp b/lib/Support/APFloat.cpp
index f143e6d..d07a3c9 100644
--- a/lib/Support/APFloat.cpp
+++ b/lib/Support/APFloat.cpp
@@ -1775,7 +1775,7 @@
   // If the exponent is large enough, we know that this value is already
   // integral, and the arithmetic below would potentially cause it to saturate
   // to +/-Inf.  Bail out early instead.
-  if (exponent+1 >= (int)semanticsPrecision(*semantics))
+  if (category == fcNormal && exponent+1 >= (int)semanticsPrecision(*semantics))
     return opOK;
 
   // The algorithm here is quite simple: we add 2^(p-1), where p is the