Rearrange some private op flag constants

Rearrange some private op flag constants

Some constants in op.h were a bit muddled up and were not grouped
according to which ops used them. And one of the comments was wrong.
The history is a bit involved:

Commit 7a52d87 added this comment, which was correct:

+  /* OP_RV2CV only */
 #define OPpENTERSUB_AMPER	8	/* Used & form to call. */

Commit 9675f7a added this constant:

   /* OP_RV2CV only */
 #define OPpENTERSUB_AMPER	8	/* Used & form to call. */
+#define OPpENTERSUB_NOPAREN	128	/* bare sub call (without parens) */

Commit cd06dff added this one, which is confusing, as it is only used
on entersub:

   /* OP_RV2CV only */
 #define OPpENTERSUB_AMPER	8	/* Used & form to call. */
 #define OPpENTERSUB_NOPAREN	128	/* bare sub call (without parens) */
+#define OPpENTERSUB_INARGS	4	/* Lval used as arg to a sub. */

Commit e26df76 added this, resulting in there being two
OP_RV2CV sections:

+  /* OP_RV2CV only */
+#define OPpMAY_RETURN_CONSTANT	1	/* If a constant sub, return the constant */
+

To top it all, commit b900987 unfortunately ‘fixed’ a mislead-
ing comment:

-  /* OP_RV2CV only */
+  /* OP_ENTERSUB and OP_RV2CV only */
 #define OPpENTERSUB_AMPER	8	/* Used & form to call. */
 #define OPpENTERSUB_NOPAREN	128	/* bare sub call (without parens) */
 #define OPpENTERSUB_INARGS	4	/* Lval used as arg to a sub. */