RISC-V: Add check for types without insn reservations

Now that all insns are guaranteed to have a type, ensure every insn
is associated with a cpu unit/insn reservation.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_sched_variable_issue): add disabled assert

Signed-off-by: Edwin Lu <ewlu@rivosinc.com>