Prevent representation inference from double-visiting.

This prevents representation inference from pushing the current value
into the worklist while it is still being worked on. This might lead
to having a value in the worklist that isn't flexible anymore.

R=verwaest@chromium.org
TEST=mjsunit/compiler/escape-analysis (+GC-Stress)

Review URL: https://codereview.chromium.org/23452020

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16546 ce2b1a6d-e550-0410-aec6-3dcde31c8c00