RISC-V: Add assert for insn operand out of range access [PR117878][NFC]

RISC-V: Add assert for insn operand out of range access [PR117878][NFC]

According to the the initial analysis of PR117878, the ice comes from
the out-of-range operand access for recog_data.operand[].  Thus, add
one assert here to expose this explicitly.

	PR target/117878

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (vlmax_avl_type_p): Add assert for
	out of range access.
	(nonvlmax_avl_type_p): Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>