Deleting jump tables and CFG

[Jan Hubicka]

>     I went into assumption that GGC thinks that 'u' does not point into
>     RTL not accessible via normal INSN chain, but it does not appear to be
>     the case, 'u' is not handled specially by ggc_mark_rtx_children, so I
>     guess this is some kind of memory corruption.
> 
> I went through the same process.  But something is blowing up and it
> definitely appears related to GC, so I'll have to investigate further.
> But the root of the problem is clearly that the CODE_LABEL isn't in the insn
> chain while the jumptable is.
That should not be major problem assuming that problematic jumptable
gets elliminated before final pass (so we don't get unreferenced
labels).
GGC has interesting behaviour when dealing with doubly linked chains
that are not really doubly linked (as in the case of dead CODE_LABEL
whose pointers point to the middle of RTL stream), perhaps you need to
manage NEXT_INSN/PREV_INSN of that CODE_LABEL to be NULL.
RTL_NEXT contains some tricks to get around this problem, so it may not
be the case anyway...

Honza