Remove definition of pipe operator

Summary: OCaml 4.00 defines the pipe operator, and the compiler is able to
optimize it so as to avoid creating a closure:

http://www.ocamlpro.com/blog/2012/08/20/ocamlpro-and-4.00.0.html

So we should remove our own implementation, and hopefully get some small speed
benefit in the process.

Reviewed By: @pathsny

Differential Revision: D1590919
Make status flags explicit in VASM.

Summary: Add a "status flags" class of virtual registers, and explicitly encode
uses/defs for all VASM instructions which affect status flags.  This
provides a mechanism for making dependencies between instructions that
set status flags and those which consume them.  The immediate
application is for translation from VASM to LLVM IR, but this also
enables various optimizations (code motion, strength reduction, etc.).

Remove the unused rorqi from VASM rather than converting it to
incorporate a status flags register.

Reviewed By: @edwinsmith

Differential Revision: D1578876