agx: Only use nest by 1 for loops w/o continue

agx: Only use nest by 1 for loops w/o continue

Apple doesn't do this, but it should be equivalent and it makes it easier to see
that we can use while_icmp for break_if_icmp in loops that don't use continue
(which Apple does do). So, the effect of this commit is to use while_icmp for
most breaks, which saves an instruction.

   total instructions in shared programs: 1764199 -> 1764076 (<.01%)
   instructions in affected programs: 24149 -> 24026 (-0.51%)
   helped: 78
   HURT: 0
   Instructions are helped.

   total bytes in shared programs: 11609306 -> 11608322 (<.01%)
   bytes in affected programs: 164604 -> 163620 (-0.60%)
   helped: 78
   HURT: 0
   Bytes are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25052>