arm/translate-a64: add FP16 x2 ops for simd_indexed

arm/translate-a64: add FP16 x2 ops for simd_indexed

A bunch of the vectorised bitwise operations just operate on larger
chunks at a time. We can do the same for the new half-precision
operations by introducing some TWOHALFOP helpers which work on each
half of a pair of half-precision operations at once.

Hopefully all this hoop jumping will get simpler once we have
generically vectorised helpers here.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180227143852.11175-16-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>