Refine verify_expr and LHS

[Richard Kenner]

> +   /* Check operand N for being valid GIMPLE and give error MSG if not.  */
> + #define CHECK_OP(N, MSG) \
> +   do { if (TREE_CODE_CLASS (TREE_CODE (TREE_OPERAND (t, N))) != 'c'	\
> +          && !is_gimple_val (TREE_OPERAND (t, N)))			\
> +        { error (MSG); return TREE_OPERAND (t, N); }} while (0)

    Can you just make this a function? It seems generically useful.

Well, the first two lines maybe, but the last line seems awfully specialized
to me.  Where else would it be used?