target/arm/translate-a64: Don't underdecode SDOT and UDOT

target/arm/translate-a64: Don't underdecode SDOT and UDOT

In the AdvSIMD scalar x indexed element and vector x indexed element
encoding group, the SDOT and UDOT instructions are vector only,
and their opcode is unallocated in the scalar group. Correctly
UNDEF this unallocated encoding.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20190125182626.9221-8-peter.maydell@linaro.org