gallium: use unreachable instead of asserts

Avoids warnings in release builds.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

src/gallium/auxiliary/gallivm/lp_bld_intr.c

@@ -84,7 +84,7 @@ lp_format_intrinsic(char *name,
       width = 64;
       break;
    default:
-      assert(0);
+      unreachable("unexpected LLVMTypeKind");
    }
 
    if (length) {