[turbofan] Add proper conversion operators for int32<->int64.

This affects arm64 and x64. Note that we do not yet optimize
these conversions. Later we will add support for merging these
conversion operators into other operations during instruction
selection.

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

Review URL: https://codereview.chromium.org/487723002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23184 ce2b1a6d-e550-0410-aec6-3dcde31c8c00