target/arm: Add some comments in Thumb decode

Add some comments to the Thumb decoder indicating what bits
of the instruction have been decoded at various points in
the code.

This is not an exhaustive set of comments; we're gradually
adding comments as we work with particular bits of the code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20181002163556.10279-6-peter.maydell@linaro.org