Improve polymorphic loads on single slots.

Improve polymorphic loads on single slots.

If all property lookups for a polymorphic load actually result in the
same field index under all maps, we can actually emit a monomorphic load
that is guarded by a map check that verifies that the actual map is in
the set of handled maps. This also allows GVN to get rid of redundant
such map checks.

R=danno@chromium.org

Review URL: https://chromiumcodereview.appspot.com/9616014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11132 ce2b1a6d-e550-0410-aec6-3dcde31c8c00