nir/opt_sink: Move ALU with constant sources

nir/opt_sink: Move ALU with constant sources

In general, sinking ALU instructions can negatively impact register pressure,
since it extends the live ranges of the sources, although it does shrink the live range
of the destination.

However, constants do not usually contribute to register pressure. This is not a
totally true assumption, but it's pretty good in practice, since...

* constants can be rematerialized (backend-dependent)
* constants can often be inlined (ISA-dependent)
* constants can sometimes be promoted to free uniform registers (ISA-dependent)
* constants can live in scalar registers although the ALU destination might need
  a vector register (and vector registers are assumed to be much more expensive
  than scalar registers, again ISA-dependent)

So, assume that constants have zero effect on register pressure. Now consider an
ALU instruction where all but one source is a constant. Then there are two
cases:

1. The ALU instruction is moved past when its source was otherwise killed. Then
   there is no effect on register pressure, since the source live range is
   extended exactly as much as the destination live range shrinks.

2. The ALU instruction is moved down but its source is still alive where it's
   moved to. Then register pressure is improved, since the source live range is
   unchanged while the destination live range shrinks.

So, as a heuristic, we always move ALU instructions where n-1 sources are
constant. As an inevitable special case, this also (necessarily) moves unary ALU
ops, which should be beneficial by the same justification. This is not 100%
perfect but it is well-motivated. Results on AGX are decent:

   total instructions in shared programs: 1796101 -> 1795652 (-0.02%)
   instructions in affected programs: 326822 -> 326373 (-0.14%)
   helped: 800
   HURT: 371
   Inconclusive result (%-change mean confidence interval includes 0).

   total bytes in shared programs: 11805004 -> 11801424 (-0.03%)
   bytes in affected programs: 2610630 -> 2607050 (-0.14%)
   helped: 912
   HURT: 462
   Inconclusive result (%-change mean confidence interval includes 0).

   total halfregs in shared programs: 525818 -> 515399 (-1.98%)
   halfregs in affected programs: 118197 -> 107778 (-8.81%)
   helped: 2095
   HURT: 804
   Halfregs are helped.

   total threads in shared programs: 18916608 -> 18917056 (<.01%)
   threads in affected programs: 4800 -> 5248 (9.33%)
   helped: 7
   HURT: 0
   Threads are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24833>