target/arm: Implement MVE shift-by-scalar

target/arm: Implement MVE shift-by-scalar

Implement the MVE instructions which perform shifts by a scalar.
These are VSHL T2, VRSHL T2, VQSHL T1 and VQRSHL T2.  They take the
shift amount in a general purpose register and shift every element in
the vector by that amount.

Mostly we can reuse the helper functions for shift-by-immediate; we
do need two new helpers for VQRSHL.

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