Make it possible to assert that certain instance types are compatible wrt HAllocate.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 176dd9fa77a04f8c401cef6fd8da191bd736cefa..95d0661a98d78cb2f4df5b586779c2457ec6d487 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1771,6 +1771,8 @@ HValue* HGraphBuilder::BuildUncheckedStringAdd(HValue* left,
     // pass CONS_STRING_TYPE or CONS_ASCII_STRING_TYPE here, so we just use
     // CONS_STRING_TYPE here. Below we decide whether the cons string is
     // one-byte or two-byte and set the appropriate map.
+    ASSERT(HAllocate::CompatibleInstanceTypes(CONS_STRING_TYPE,
+                                              CONS_ASCII_STRING_TYPE));
     HAllocate* string = Add<HAllocate>(Add<HConstant>(ConsString::kSize),
                                        HType::String(), pretenure_flag,
                                        CONS_STRING_TYPE);