[turbofan] Fix branch hints for ChangeInt32ToTagged and ChangeTaggedToUI32.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/694353006

Cr-Commit-Position: refs/heads/master@{#25106}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25106 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
[turbofan] Add AllocateHeapNumberStub to avoid runtime call.

TEST=unittests
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/703473004

Cr-Commit-Position: refs/heads/master@{#25107}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25107 ce2b1a6d-e550-0410-aec6-3dcde31c8c00