Previously Crankshaft emitted a generic load for these, now we emit a load of a
named field, guarded by a proto chain check.

LCheckPrototypeMaps now returns the holder, which is for free, because it
already had to check its map as the last step, anyway. This is in sync with what
StubCompiler::CheckPrototype does.

Review URL: https://codereview.chromium.org/11338030

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12847 ce2b1a6d-e550-0410-aec6-3dcde31c8c00