[turbofan] Track up to 32 fields for LoadElimination.

With the current approach we cannot eliminate context accesses in
mid-size function contexts, so let's bump the limit a bit to make
sure we can optimize those as well.

R=jarin@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2182973004
Cr-Commit-Position: refs/heads/master@{#38050}

src/compiler/load-elimination.h

@@ -107,7 +107,7 @@ class LoadElimination final : public AdvancedReducer {
     ZoneMap<Node*, Node*> info_for_node_;
   };
 
-  static size_t const kMaxTrackedFields = 16;
+  static size_t const kMaxTrackedFields = 32;
 
   class AbstractState final : public ZoneObject {
    public: