agx: Schedule for register pressure

Since we register allocate in SSA, the number of registers required (register
demand) equals to the maximum number of simultaneous live values (register
pressure). So if we can reduce register pressure, we are guaranteed to reduce
register demand. Even an ineffective heuristic like randomly swapping
instructions can only reduce pressure as long as it's conservative.

This implements one such heuristic: in each block, schedule backwards, selecting
the free instruction that looks like it will reduce liveness the most. In other
words, the greedy algorithm to reduce register pressure. At the end of the
block, if we haven't actually reduced pressure, we bail. This isn't optimal, but
it's well-motivated and optimally handles special cases (like 0-source
instructions).

This is based on the scheduler I originally wrote for Mali.

In my Dolphin ubershader branch, this improved performance at native 4K by 10fps
(105fps->115fps) when I measured together with some other optimizations. On top
of my current next (which notably includes nir_opt_sink improvements), this
commit alone goes (53fps->54fps) which is considerably less impressive :-p

shader-db results are a win, but not as large as we might hope. Instruction
count win seems to be from the smaller live ranges being easier on RA (fewer
swaps / moves). The two shaders affected for thread count are from fifa mobile,
which go from 640 threads ->
1024 (full occupancy). In other words... this heuristic does an excellent job in
a small subset of shaders. The Dolphin ubershader win was real, though :~)

Note these shader-db wins are on top of a branch with the nir_opt_sink
improvements. Without that, the stats are much better... The schedulers have
some overlap, but they're better together.

   total instructions in shared programs: 1766635 -> 1763496 (-0.18%)
   instructions in affected programs: 445855 -> 442716 (-0.70%)
   helped: 1963
   HURT: 350
   Instructions are helped.

   total bytes in shared programs: 11597648 -> 11586924 (-0.09%)
   bytes in affected programs: 3106230 -> 3095506 (-0.35%)
   helped: 2003
   HURT: 374
   Bytes are helped.

   total halfregs in shared programs: 504609 -> 481980 (-4.48%)
   halfregs in affected programs: 138322 -> 115693 (-16.36%)
   helped: 3405
   HURT: 311
   Halfregs are helped.

   total threads in shared programs: 18839936 -> 18840704 (<.01%)
   threads in affected programs: 1280 -> 2048 (60.00%)
   helped: 2
   HURT: 0

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

src/asahi/compiler/agx_compiler.h

@@ -759,6 +759,7 @@ void agx_lower_pseudo(agx_context *ctx);
 void agx_lower_uniform_sources(agx_context *ctx);
 void agx_opt_cse(agx_context *ctx);
 void agx_dce(agx_context *ctx, bool partial);
+void agx_pressure_schedule(agx_context *ctx);
 void agx_ra(agx_context *ctx);
 void agx_lower_64bit_postra(agx_context *ctx);
 void agx_insert_waits(agx_context *ctx);