target/arm: Implement MVE incrementing/decrementing dup insns

target/arm: Implement MVE incrementing/decrementing dup insns

Implement the MVE incrementing/decrementing dup insns VIDUP, VDDUP,
VIWDUP and VDWDUP.  These fill the elements of a vector with
successively incrementing values, starting at the offset specified in
a general purpose register.  The final value of the offset is written
back to this register.  The wrapping variants take a second general
purpose register which specifies the point where the count should
wrap back to 0.

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