add Op(MORE|LAST|MAYBE)SIB_set; rm OpSIBLING_set

add Op(MORE|LAST|MAYBE)SIB_set; rm OpSIBLING_set

the OpSIBLING_set() macro just set the op_sibling/op_sibparent field,
and didn't update op_moresib.

Remove this macro, and replace it with the three macros

    OpMORESIB_set
    OpLASTSIB_set
    OpMAYBESIB_set

which also set op_moresib appropriately. These were suggested by Zefram.

Then in the remaining areas in op.c where low-level op_sibling/op_moresib
tweaking is done, use the new macros instead (so if nothing else, they get
used and tested.)