target-arm: Use sextract32() in branch decode

target-arm: Use sextract32() in branch decode

In the decode of ARM B and BL insns, swap the order of the
"append 2 implicit zeros to imm24" and the sign extend, and
use the new sextract32() utility function to do the latter.
This avoids a direct dependency on the undefined C behaviour
of shifting into the sign bit of an integer.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1378391908-22137-2-git-send-email-peter.maydell@linaro.org