Add fixed-non-allocatable operand support

[Amanieu]

This allows a non-allocatable PReg to be passed on directly to the allocations vector without any liverange tracking from the register allocator. The main intended use case is to support ISA-specific special registers such as a fixed zero register.

Fixes #3

[Amanieu]

I got a 6% speedup in compilation time by replacing my uses of the zero register with Operand::fixed_nonallocatable. I was previously defining the zero register using Operand::reg_fixed_def on the entry instruction, which was subtly incorrect since regalloc could sometimes emit a move that would write to the zero register.

[cfallin]

Looks reasonable to me; thanks!