i965/fs.h: Refactor tests for instructions modifying a register.

i965/fs.h: Refactor tests for instructions modifying a register.

There's one instance of a potential behavior change: propagate_constants may
now propagate into a part of a vgrf after a different part of it was
overwritten by a send that returns multiple registers.  I don't think we ever
generate IR that meets that condition, but it's something to note if we bisect
behavior change to this.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>