Emit uses of ZR intelligently on ARM

Summary: I added this to the immediate-folding pass. It seemed like most of the
unfortunate zero constants came from stores (e.g. in SpillFrame) so I've
added those for now.

In the process of doing this, I found a couple places where we were
emitting "add x0, x0, #0x0" which is totally useless, so I eliminated
those.

I eliminated all mention of zr from code-gen-arm.cpp, since we should
just make this pass handle all of it.

Reviewed By: @edwinsmith

Differential Revision: D1629590
Enable PGO for all functions

Summary: In server mode, only functions marked as hot were target of PGO.
This diff enables PGO for all functions.

Reviewed By: @jdelong

Differential Revision: D1623210