llvmpipe: change LP_MAX_SHADER_INSTRUCTIONS definition

llvmpipe: change LP_MAX_SHADER_INSTRUCTIONS definition

This change will double cache size for branches which have a lower
LP_MAX_SHADER_VARIANTS limit (it will not do anything on master).
The reason is that nowadays shaders tend to be quite a bit larger than they
were (they were big when llvmpipe didn't have a fs loop, got much smaller with
that loop, and since then have gradually increased quite a bit though still
smaller than without the fs loop for various reasons - among them being d3d10
compliance, usage of 8-wide vectors, non-swizzled blend code). Thus effectively
less shaders would be cached (unless they were very small and the variant limit
was hit first). Also, since we're getting rid of the IR nowadays, the cached
shaders shouldn't need all that much memory actually.