target/arm: Convert Neon VEXT to decodetree

target/arm: Convert Neon VEXT to decodetree

Convert the Neon VEXT insn to decodetree. Rather than keeping the
old implementation which used fixed temporaries cpu_V0 and cpu_V1
and did the extraction with by-hand shift and logic ops, we use
the TCG extract2 insn.

We don't need to special case 0 or 8 immediates any more as the
optimizer is smart enough to throw away the dead code.

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