radv: enable lower_sub to fix loop unrolling.

Loop unroll asserts if it hits a sub, we don't really want
to lower subs as llvm handles these things, but do this for
now, until we can fix loop unroll to work with subs.

Fixes: 14ae0bfa5 (radv: Add NIR loop unrolling)
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/amd/vulkan/radv_pipeline.c

@@ -52,6 +52,7 @@ static const struct nir_shader_compiler_options nir_options = {
 	.lower_flrp32 = true,
 	.lower_fsat = true,
 	.lower_fdiv = true,
+	.lower_sub = true,
 	.lower_pack_snorm_2x16 = true,
 	.lower_pack_snorm_4x8 = true,
 	.lower_pack_unorm_2x16 = true,