MIPS/opcodes: Properly handle ISA exclusion

Remove the hack used for MIPSr6 ISA exclusion from `cpu_is_member' and
handle the exclusion for any ISA levels properly in `opcode_is_member'.
Flatten the structure of the `if' statements there.  No functional
change for the existing opcode tables.

	include/
	* opcode/mips.h (cpu_is_member): Remove code for MIPSr6 ISA
	exclusion.
	(opcode_is_member): Handle ISA level exclusion.
2 files changed