target/arm: Implement new VFP fp16 insn VINS

target/arm: Implement new VFP fp16 insn VINS

The fp16 extension includes a new instruction VINS, which copies the
lower 16 bits of a 32-bit source VFP register into the upper 16 bits
of the destination.  Implement it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200828183354.27913-20-peter.maydell@linaro.org