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..3fec8504935cf742096c04e75184ba2135ef4b1c 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -1771,9 +1771,11 @@ 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.
-    HAllocate* string = Add<HAllocate>(Add<HConstant>(ConsString::kSize),
-                                       HType::String(), pretenure_flag,
-                                       CONS_STRING_TYPE);
+    ASSERT_EQ(HAllocate::ComputeFlags(pretenure_flag, CONS_STRING_TYPE),
+              HAllocate::ComputeFlags(pretenure_flag, CONS_ASCII_STRING_TYPE));
+    HAllocate* string = Add<HAllocate>(
+        Add<HConstant>(ConsString::kSize), HType::String(),
+        HAllocate::ComputeFlags(pretenure_flag, CONS_STRING_TYPE));
 
     // Compute the intersection of instance types.
     HValue* anded_instance_types = AddUncasted<HBitwise>(