Shift off both GC bits in OCaml repr shared heap header
Shift off both GC bits in OCaml repr shared heap header

Summary:
This removes a shift from mk_header and obj_tag, and is just a bit simpler
overall while losing nothing.

Reviewed By: nmote

Differential Revision: D25447327

fbshipit-source-id: c6e7107c47a97e080d836fcac55f2e8d7025a40a

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