Convert arm::CodeGenerator to use vregs and ssa.

Summary: Moved assignRegs() from back-end-x64.cpp to reg-alloc.cpp, made it
portable by plumbing storesCell() and loadsCell() through BackEnd.

Moved cond() to code-gen-helpers.h, before realizing I didn't need
it for arm. But that's an ok target-independent place for it anyway.

Removed dead cgShuffle, push/popregs instructions, and RegSaver.

Reviewed By: @alexmalyshev

Differential Revision: D1582700