target/arm: Reject dup_i w/ shifted byte early

Remove the unparsed extraction in trans_DUP_i,
which is intended to reject an 8-bit shift of
an 8-bit constant for 8-bit element.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-72-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>