target/arm: Implement MVE VQADD and VQSUB

target/arm: Implement MVE VQADD and VQSUB

Implement the MVE VQADD and VQSUB insns, which perform saturating
addition of a scalar to each element.  Note that individual bytes of
each result element are used or discarded according to the predicate
mask, but FPSCR.QC is only set if the predicate mask for the lowest
byte of the element is set.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20210617121628[email protected]