Turn Runtime_MigrateInstance into Runtime_TryMigrateInstance

because it must not cause lazy deopts because it is called from deferred code that cannot handle lazy deopts.

Hat tip to Ben for doing most of the debugging work, and to Toon for writing the regression test.

BUG=chromium:315252
LOG=Y
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/131243003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18586 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Delete useless --loop-weight flag

killing a static variable in the process.

BUG=chromium:333871
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/131223009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18587 ce2b1a6d-e550-0410-aec6-3dcde31c8c00