[SystemZ] Allow 8-bit operands to RISBG

RISBG has three 8-bit operands (I3, I4 and I5).  I'd originally
restricted all three to 6 bits, since that's the only range we intended
to use at the time.  However, the top bit of I4 acts as a "zero" flag for
RISBG, while the top bit of I3 acts as a "test" flag for RNSBG & co.
This patch therefore allows them to have the full 8-bit range.
I've left the fifth operand as a 6-bit value for now since the
upper 2 bits have no defined meaning.