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
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