Simplify KeysetIterEnd and BespokeIterEnd on static arrays to Count

Summary:
Static arrays do not have tombstones. Without tombstones, IterEnd is the same
as Count, so simplify them to Count, which we likely already loaded for other
reasons.

This optimizes away tombstone detection, as it will turn IterEnd vs Count
comparison to Count vs Count, where GVN will deduplicate these counts and
simplifier will then optimize away of EqInt on two same SSATmps.

Differential Revision: D57466215

fbshipit-source-id: 9398eb797697691640295aa92ae5f64dcc042038

hphp/runtime/vm/jit/simplify.cpp

@@ -3247,6 +3247,9 @@ SSATmp* simplifyKeysetIterEnd(State& env, const IRInstruction* inst) {
     return cns(env, keyset->iterLimit());
   }
 
+  // Static keysets do not have tombstones.
+  if (arr->isA(TStaticKeyset)) return gen(env, CountKeyset, arr);
+
   return nullptr;
 }
 
@@ -3403,6 +3406,9 @@ SSATmp* simplifyBespokeIterEnd(State& env, const IRInstruction* inst) {
     return gen(env, CountDict, arr);
   }
 
+  // Static arrays do not have tombstones.
+  if (arr->isA(TStaticArrLike)) return gen(env, Count, arr);
+
   return nullptr;
 }