target/arm: Implement vector shifted FCVT for fp16

target/arm: Implement vector shifted FCVT for fp16

While we have some of the scalar paths for FCVT for fp16,
we failed to decode the fp16 version of these instructions.

Cc: qemu-stable@nongnu.org
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180502221552.3873-3-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
(cherry picked from commit d0ba8e74acd299b092786ffc30b306638d395a9e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>