Deleting jump tables and CFG

[Richard Kenner]

    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.