[ARM,MVE] Fix valid immediate range for vsliq_n.

In common with most MVE immediate shift instructions, the left shift
takes an immediate in the range [0,n-1], while the right shift takes
one in the range [1,n]. I had absent-mindedly made them both the
latter.

While I'm here, I've added a set of regression tests checking both
ends of the immediate range for a representative sample of the
immediate shifts.