v3d: enable uniform expression propagation from outputs to the next shader

This will take effect after nir_opt_varyings is enabled by another MR, and
will fix existing shader compiler crashes thanks to better optimizations.

For example, one GLSL program that failed to compile and had 226 VS
instructions and 356 FS instructions in NIR will be reduced to 2 or 3
instructions per shader.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31942>

src/gallium/drivers/v3d/v3d_screen.c

@@ -762,6 +762,7 @@ v3d_screen_get_compiler_options(struct pipe_screen *pscreen,
                 .max_unroll_iterations = 16,
                 .force_indirect_unrolling_sampler = true,
                 .scalarize_ddx = true,
+                .max_varying_expression_cost = 4,
         };
 
         if (!initialized) {