SILOptimizer: Workaround for suspect SubstitutionMap usage

Author: slavapestov

lib/SILOptimizer/Utils/Generics.cpp

@@ -1399,7 +1399,8 @@ class FunctionSignaturePartialSpecializer {
       SubstitutionMap::get(
         SpecializedGenericSig,
         [&](SubstitutableType *type) -> Type {
-          return CalleeGenericEnv->mapTypeIntoContext(type);
+          return CalleeGenericEnv->mapTypeIntoContext(
+              SpecializedGenericSig.getReducedType(type));
         },
         LookUpConformanceInModule());
   }