[compiler] Perform MapRef::prototype concurrently

We can keep the non-atomic accessors for read/write since we set the
prototype on the map at initialization.

Bug: v8:7790, chromium:1150811
Change-Id: Ied7763c87a71c6aa93099dec3405873ab7419643
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2773052
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73615}

src/compiler/js-heap-broker.cc

@@ -3518,7 +3518,7 @@ BIMODAL_ACCESSOR_WITH_FLAG_B(Map, bit_field, is_undetectable,
 BIMODAL_ACCESSOR_C(Map, int, instance_size)
 BIMODAL_ACCESSOR_C(Map, int, NextFreePropertyIndex)
 BIMODAL_ACCESSOR_C(Map, int, UnusedPropertyFields)
-BIMODAL_ACCESSOR(Map, HeapObject, prototype)
+BIMODAL_ACCESSOR_WITH_FLAG(Map, HeapObject, prototype)
 BIMODAL_ACCESSOR_WITH_FLAG_C(Map, InstanceType, instance_type)
 BIMODAL_ACCESSOR_WITH_FLAG(Map, Object, GetConstructor)
 BIMODAL_ACCESSOR_WITH_FLAG(Map, HeapObject, GetBackPointer)