[RISCV] Make getInstSeqCost handle other Zb* instructions.

We haven't been updating this as Zb* instructions have been used
for immediate materialization. They will hit the default case and
trigger an llvm_unreachable. Instead of trying to list them all,
assume instructions that aren't explicitly listed aren't compressible.

Spotted while looking at integer materialization for other reasons.
I haven't seen a crash from this yet.