[SystemZ] Avoid using i8 constants for immediate fields

Immediate fields that have no natural MVT type tended to use i8 if the
field was small enough.  This was a bit confusing since i8 isn't a legal
type for the target.  Fields for short immediates in a 32-bit or 64-bit
operation use i32 or i64 instead, so it would be better to do the same
for all fields.

No behavioral change intended.