agx: Omit push_exec at top level

agx: Omit push_exec at top level

In general, loops need a push_exec at the start for correctness. However, a
push_exec at the top level (non-nested) is a no-op, so we can omit and save a
few cycles.

   total instructions in shared programs: 1764350 -> 1764311 (<.01%)
   instructions in affected programs: 7339 -> 7300 (-0.53%)
   helped: 36
   HURT: 0
   Instructions are helped.

   total bytes in shared programs: 11610212 -> 11609978 (<.01%)
   bytes in affected programs: 48638 -> 48404 (-0.48%)
   helped: 36
   HURT: 0
   Bytes are helped.

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