etnaviv: Add unreachable statement to etna_amode to fix compilation warnings

Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/gallium/drivers/etnaviv/etnaviv_compiler.c

@@ -885,6 +885,8 @@ etna_amode(struct tgsi_ind_register indirect)
    default:
       assert(!"Invalid swizzle");
    }
+
+   unreachable("bad swizzle");
 }
 
 /* convert destination operand */