target/s390x: Use tcg_gen_gvec_dup_imm

target/s390x: Use tcg_gen_gvec_dup_imm

The gen_gvec_dupi switch is unnecessary with the new function.
Replace it with a local gen_gvec_dup_imm that takes care of the
register to offset conversion and length arguments.

Drop zero_vec and use use gen_gvec_dup_imm with 0.

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