[csa] Fix type casing in GetProperty

The checked node and used node were mismatched. It checked if
"var_unique" is a string, but it used "key" which may not be a string.

Bug: v8:4911, v8:7161, chromium:800077
Change-Id: Ia2aee8b77ac33500430365a4800bf9cca40a28fc
Reviewed-on: https://chromium-review.googlesource.com/855138
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50435}

src/code-stub-assembler.cc

@@ -7134,7 +7134,8 @@ void CodeStubAssembler::TryPrototypeChainLookup(
         GotoIfNot(InstanceTypeEqual(holder_instance_type, JS_TYPED_ARRAY_TYPE),
                   &next_proto);
         GotoIfNot(IsString(var_unique.value()), &next_proto);
-        BranchIfMaybeSpecialIndex(CAST(key), if_bailout, &next_proto);
+        BranchIfMaybeSpecialIndex(CAST(var_unique.value()), if_bailout,
+                                  &next_proto);
       }
 
       BIND(&next_proto);

test/mjsunit/regress/regress-crbug-800077.js

@@ -0,0 +1,6 @@
+// Copyright 2018 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+var sample = new Float64Array(1);
+Reflect.has(sample, undefined);