MIPS/opcodes: Factor out ISA matching against flags

MIPS/opcodes: Factor out ISA matching against flags

In preparation for the next change factor out code for ISA matching
against instruction flags used in MIPS opcode tables, similarly to how
CPU matching is already done.  No functional change, though for clarity
split the single `if' statement into multiple ones and use temporaries
rather than repeated expressions.

	include/
	* opcode/mips.h (isa_is_member): New inline function, factored
	out from...
	(opcode_is_member): ... here.