RequirementMachine: Pack RewriteStep more tightly

Author: slavapestov

lib/AST/RequirementMachine/RewriteLoop.h

@@ -219,11 +219,11 @@ struct RewriteStep {
 
   /// The size of the left whisker, which is the position within the term where
   /// the rule is being applied. In A.(X => Y).B, this is |A|=1.
-  unsigned StartOffset : 16;
+  unsigned StartOffset : 13;
 
   /// The size of the right whisker, which is the length of the remaining suffix
   /// after the rule is applied. In A.(X => Y).B, this is |B|=1.
-  unsigned EndOffset : 16;
+  unsigned EndOffset : 13;
 
   /// If Kind is Rule, the index of the rule in the rewrite system.
   ///