op.c: change Optype to I32 for cmpchain functions

op.c: change Optype to I32 for cmpchain functions

Optype appears to be almost completely unused, and on Win32 builds
we saw warnings from the cmpchain patches:

    perly.y(1063) : warning C4244: 'function' : conversion from 'I32' to 'Optype', possible loss of data
    perly.y(1065) : warning C4244: 'function' : conversion from 'I32' to 'Optype', possible loss of data
    perly.y(1079) : warning C4244: 'function' : conversion from 'I32' to 'Optype', possible loss of data
    perly.y(1081) : warning C4244: 'function' : conversion from 'I32' to 'Optype', possible loss of data

Reviewing the code I noticed that functions like Perl_newBINOP() have
an I32 type argument, and functions like OpTYPE_set() coerce such
arguments into type OPCODE:

	#define OpTYPE_set(o,type) \
	STMT_START {                                \
	    o->op_type = (OPCODE)type;              \
	    o->op_ppaddr = PL_ppaddr[type];         \
	} STMT_END

this patch changes the signature to the new cmpchain functions so that
they do they same, and change the type for storage for op_type values
to also use OPCODE like most of the other op.c code.