regcomp.c: Extract duplicated code to fcn

regcomp.c: Extract duplicated code to fcn

This causes the nearly-duplicate code of S_reg_node and S_reganode to be
placed into a single function, S_regnode_guts.

There is one place where it might not be obvious that this doesn't
change things.  And that is under DEBUGGING, reg_node() called

    Set_Node_Offset(RExC_emit, RExC_parse + (op == END));

and reganode called

    Set_Cur_Node_Offset;

However Set_Cur_Node_Offset is defined to be

    Set_Node_Offset(RExC_emit, RExC_parse)

and since op will never be END for reganode, the two statements are
equivalent.