This optimizes Map#get and Map#has for the case where the key is known
to be a JSReceiver. This generalizes the existing logic for the
FindOrderedHashMapEntryForSigned32Key operator to also deal with
receivers. This gives a nice 33% boost on the map-set-lookup-es6 test
of the six-speed benchmark suite.

Drive-by-fix: Rename the FindOrderedHashMapEntryForInt32Key operator to
FindOrderedHashMapEntryForSigned32Key to match the naming of the types.

R=jarin@chromium.org

Bug: v8:5267, v8:7001
Change-Id: Ifab8414f26adee7ec833d8cb94ae0ac49f2c3d35
Reviewed-on: https://chromium-review.googlesource.com/738180Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48938}