target/arm: Use dup_const() instead of bitfield_replicate()

Use dup_const() instead of bitfield_replicate() in
disas_simd_mod_imm().

(We can't replace the other use of bitfield_replicate() in this file,
in logic_imm_decode_wmask(), because that location needs to handle 2
and 4 bit elements, which dup_const() cannot.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210628135835.6690-6-peter.maydell@linaro.org