target/arm: Note that we handle VMOVL as a special case of VSHLL

target/arm: Note that we handle VMOVL as a special case of VSHLL

Although the architecture doesn't define it as an alias, VMOVL
(vector move long) is encoded as a VSHLL with a zero shift.
Add a comment in the decode file noting that we handle VMOVL
as part of VSHLL.

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