x86: merge/move logic determining the EVEX disp8 shift

x86: merge/move logic determining the EVEX disp8 shift

Fold redundant case blocks and move the extra adjustments logic into
the single case block that actually needs it - there's no need to go
through the extra logic for all the other cases. Also utilize there that
vex.b cannot be set at this point, due to earlier logic. Reduce the
comment there, which was partly stale anyway.