target/arm: Swap argument order for VSHL during decode

target/arm: Swap argument order for VSHL during decode

Rather than perform the argument swap during code generation,
perform it during decode.  This means it doesn't have to be
special cased later, and we can share code with aarch64 code
generation.  Hopefully the decode comment addresses any confusion
that might arise in between.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200513163245.17915-9-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>