Implement the MVE VBRSR insn, which reverses a specified
number of bits in each element, setting the rest to zero.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210617121628.20116-26-peter.maydell@linaro.org