[CSA] Fix IsString.

IsString should not count FIRST_NON_STRING_TYPE as a string type.

BUG=

Change-Id: I093945763c3d2b56de5b4b5745ba207fe2d8b5d1
Reviewed-on: https://chromium-review.googlesource.com/453899Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43783}

src/code-stub-assembler.cc

@@ -2947,8 +2947,8 @@ Node* CodeStubAssembler::IsName(Node* object) {
 }
 
 Node* CodeStubAssembler::IsString(Node* object) {
-  return Int32LessThanOrEqual(LoadInstanceType(object),
-                              Int32Constant(FIRST_NONSTRING_TYPE));
+  return Int32LessThan(LoadInstanceType(object),
+                       Int32Constant(FIRST_NONSTRING_TYPE));
 }
 
 Node* CodeStubAssembler::IsSymbol(Node* object) {