tcg: Add gvec expanders for variable shift

tcg: Add gvec expanders for variable shift

The gvec expanders perform a modulo on the shift count.  If the target
requires alternate behaviour, then it cannot use the generic gvec
expanders anyway, and will have to have its own custom code.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>