tcg/aarch64: Use tcg_constant_vec with tcg vec expanders

Improve rotrv_vec to reduce "t1 = -v2, t2 = t1 + c" to
"t1 = -v2, t2 = c - v2".  This avoids a serial dependency
between t1 and t2.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>