target-arm: Fix decoding of Thumb preload and hint space

target-arm: Fix decoding of Thumb preload and hint space

Refine the decoding of the Thumb preload and hint space, so we
UNDEF on the patterns that are supposed to UNDEF rather than NOP.
We also move the tests for this space earlier, so we don't emit
harmless but unnecessary address generation code for preload
hints (which by their nature are likely to be in hot code paths).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>