agx: Optimize out pointless else instructions

agx: Optimize out pointless else instructions

Now that they're in the right blocks, this is easy. Includes an informal proof
and the implementation itself is built around a finite state machine, which
together meant this code worked on its first try :~)

And hey, it's a pointless little instruction saving optimization I've wanted to
do for a while~

Major note is that this HAS to be done after register allocation, since it
doesn't update the control flow graph and would introduce critical edges
if it tried to actually deleted the else block. The intuitive reason for this is
simple: sometimes RA needs to insert instructions into the else block, even if
it was empty in the original NIR, so we always need an else block even if we can
delete it with this pass after RA.

   total instructions in shared programs: 1778390 -> 1776725 (-0.09%)
   instructions in affected programs: 268459 -> 266794 (-0.62%)
   helped: 1013
   HURT: 0
   Instructions are helped.

   total bytes in shared programs: 12185102 -> 12175112 (-0.08%)
   bytes in affected programs: 1927524 -> 1917534 (-0.52%)
   helped: 1013
   HURT: 0
   Bytes are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24635>