broadcom/compiler: don't emit redundant ldunif

If we emit a new uniform and that uniform has already been emitted
in the same block we can just reuse that.

There is a balancing game here between reducing ldunif instructions
and not increasing register pressure too much though, so we put
a limit to how far back we are willing to look for a previous
definition of the uniform. Based on shader-db results, 20 instructions
produces best results.

total instructions in shared programs: 14928266 -> 14907432 (-0.14%)
instructions in affected programs: 6431841 -> 6411007 (-0.32%)
helped: 15270
HURT: 10772
Instructions are helped.

total uniforms in shared programs: 3944672 -> 3840276 (-2.65%)
uniforms in affected programs: 1827184 -> 1722788 (-5.71%)
helped: 30423
HURT: 845
Uniforms are helped.

total inst-and-stalls in shared programs: 14957813 -> 14936873 (-0.14%)
inst-and-stalls in affected programs: 6475349 -> 6454409 (-0.32%)
helped: 15287
HURT: 10852
Inst-and-stalls are helped.

v2 (Eric):
 - consider ldunifrf too
 - check that no other instruction writes to the register

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9077>

src/broadcom/compiler/v3d_compiler.h

@@ -643,6 +643,13 @@ struct v3d_compile {
          */
         bool disable_tmu_pipelining;
 
+        /* Emits ldunif for each new uniform, even if the uniform was already
+         * emitted in the same block. Useful to compile shaders with high
+         * register pressure or to disable the optimization during uniform
+         * spills.
+         */
+        bool disable_ldunif_opt;
+
         /* State for whether we're executing on each channel currently.  0 if
          * yes, otherwise a block number + 1 that the channel jumped to.
          */