Register Allocation (x64 Backend)

# Register Allocation (x64 Backend)

`HostLoc`s contain values. A `HostLoc` ("host value location") is either a host CPU register or a host spill location.

Values once set cannot be changed. Values can however be moved by the register allocator between `HostLoc`s. This is
handled by the register allocator itself and code that uses the register allocator need not and should not move values
between registers.

The register allocator is based on three concepts: `Use`, `Def` and `Scratch`.

* `Use`: The use of a value.
* `Define`: The definition of a value, this is the only time when a value is set.
* `Scratch`: Allocate a register that can be freely modified as one wishes.

Note that `Use`ing a value decrements its `use_count` by one. When the `use_count` reaches zero the value is discarded and no longer exists.

The member functions on `RegAlloc` are just a combination of the above concepts.

### `Scratch`

    Xbyak::Reg64 ScratchGpr(HostLocList desired_locations = any_gpr)
    Xbyak::Xmm ScratchXmm(HostLocList desired_locations = any_xmm)

At runtime, allocate one of the registers in `desired_locations`. You are free to modify the register. The register is discarded at the end of the allocation scope.