When typing phi nodes, weaken union of integer constants to a range.

src/types.h

Index: src/types.h
diff --git a/src/types.h b/src/types.h
index 84fbe38776aa29cd94d60aef3e6066b2e02150de..fdd45bf393042378eef35959a8aa611c13c248bb 100644
--- a/src/types.h
+++ b/src/types.h
@@ -95,7 +95,8 @@ namespace internal {
 // RANGE TYPES
 //
 // A range type represents a continuous integer interval by its minimum and
-// maximum value.  Either value might be an infinity.
+// maximum value.  Either value may be an infinity, in which case that infinity
+// itself is also included in the range.   A range never contains NaN or -0.
 //
 // Constant(v) is considered a subtype of Range(x..y) if v happens to be an
 // integer between x and y.
@@ -513,9 +514,8 @@ class TypeImpl : public Config::Base {
   double Min();
   double Max();
 
-  // Extracts a range from the type. If the type is a range, it just
-  // returns it; if it is a union, it returns the range component.
-  // Note that it does not contain range for constants.
+  // Extracts a range from the type: if the type is a range or a union
+  // containing a range, that range is returned; otherwise, NULL is returned.
   RangeType* GetRange();
 
   int NumClasses();