Bit shuffle operations can be written with very few TCG instructions
(between 5 and 8), so it is worth converting them to TCG.

This code also move all bit shuffle generation code to a separate
function in order to have a cleaner exception code path, that is it
doesn't store back the TCG register to the target register after the
exception, as the TCG register doesn't exist anymore.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5679 c046a42c-6fe2-441c-8c8c-71466251a162