Idea: Keep track of set condition codes

[eira-fransham]

If we kept track of what conditions were set we could get better performance on the idiom:

for (int i = something; i != 0; i--) {
  // ...
}

Instead of emitting a cmp after the dec, we can work out that we already have the zero flag set from the dec and just emit a jnz. Currently there's no way to abstract over different instructions and get metadata for them, so this would be very complicated to implement without a new assembler.

EDIT: Keeping track of what instructions mutate which condition codes would allow us to implement CraneStation/lightbeam#16 in a way that only spills the "condition" if we're about to emit an instruction that would invalidate it.

[alexcrichton]

Lightbeam was removed in #3390 as explained in RFC 14, so I'm going to close this.