broadcom/compiler: adjust register threshold for 2-thread compiles

We have twice the registers in this case so it makes sense to double
this as well. While this causes slight regressions in shader-db
stats (due to additional register pressure), it helps us hide latency
of memory reads better on 2-thread compiles, where the thread switch
mechanism will be less effective. This shows a ~3% performance
improvement on the UE4 SunTemple demo.

total instructions in shared programs: 12642413 -> 12656164 (0.11%)
instructions in affected programs: 2272652 -> 2286403 (0.61%)
helped: 2924
HURT: 3389

total uniforms in shared programs: 3703861 -> 3704776 (0.02%)
uniforms in affected programs: 213729 -> 214644 (0.43%)
helped: 823
HURT: 1272

total max-temps in shared programs: 2150686 -> 2153505 (0.13%)
max-temps in affected programs: 191332 -> 194151 (1.47%)
helped: 1900
HURT: 1891

total spills in shared programs: 3255 -> 3274 (0.58%)
spills in affected programs: 166 -> 185 (11.45%)
helped: 3
HURT: 6

total fills in shared programs: 4630 -> 4656 (0.56%)
fills in affected programs: 367 -> 393 (7.08%)
helped: 7
HURT: 15

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15276>

src/broadcom/compiler/vir.c

@@ -1595,7 +1595,7 @@ v3d_attempt_compile(struct v3d_compile *c)
                 /* Schedule for about half our register space, to enable more
                  * shaders to hit 4 threads.
                  */
-                .threshold = 24,
+                .threshold = c->threads == 4 ? 24 : 48,
 
                 /* Vertex shaders share the same memory for inputs and outputs,
                  * fragement and geometry shaders do not.