When inlining sloppy functions try to find some witness in the effect
chain that the receiver is already a JSReceiver and thereby avoid
inserting the JSConvertReceiver node, which we currently cannot really
optimize away most of the time.

Middle-term we may want to change the way CheckMaps works and have some
unified mechanism to deal with effect chain walks to find witnesses for
various map related facts. Also we may want to consider doing this
optimization later, although that requires some more refactorings since
we already promised that JSConvertReceiver gives a Type::Receiver.

R=mstarzinger@chromium.org
BUG=v8:5267

Review-Url: https://codereview.chromium.org/2333213002
Cr-Commit-Position: refs/heads/master@{#39379}