[turbofan] Make sure Operator is really immutable.

Make Operator const-correct in preparation for allocating sharing
Operators across different isolates (using LazyInstance).

TEST=cctest,compiler-unittests,mjsunit
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/539933002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23677 ce2b1a6d-e550-0410-aec6-3dcde31c8c00