For constant shifts, we can simply shift the s_mask.

For variable shifts, we know that sar does not reduce
the s_mask, which helps for sequences like

    ext32s_i64  t, in
    sar_i64     t, t, v
    ext32s_i64  out, t

allowing the final extend to be eliminated.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>