For polymorphic calls (also loads and stores) we construct a type switch
graph that has a basic block merging all the variants.  There is an
environment simulation before the goto at the end of all the predecessor
blocks.  This simulation is used to define the environment on entry to the
successor block, and captures the return value of the call.  In effect
contexts, this value should not be present in the environment.

The fix is to use the AST context to decide whether to have this value in
the join node's environment at all.

BUG=1014

Review URL: http://codereview.chromium.org/6065014

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6189 ce2b1a6d-e550-0410-aec6-3dcde31c8c00