[RISCV] Use bits<1> for AltFmt in RISCVVPseudo. (#182581)
This makes the searchable table emitter use uint8_t instead of bool in
the KeyType struct. This is needed to allow us to use a bitfield in the
PseudoInfo struct and avoid a compare between a bool and uint16_t. We
already use this same bits<1> trick in other searchable tables.
Fixes #182485
