target-alpha: instruction decoding simplification

Use a litteral value of 0 when rb31 is used. This reduces the tests
in the instruction decoding. Also remove almost unused corner cases.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5250 c046a42c-6fe2-441c-8c8c-71466251a162