nothrow changes

[Jeffrey A Law]

All the nothrow work (as a whole) has broken nonlocal gotos.

Basically we are setting TREE_NOTHROW to a nonzero value on the function decl
for the nested function that has a computed goto.

When we later expand a call to that function we see TREE_NOTHROW is set
and set the EH region on the generated CALL_INSN to -1, so flow thinks
the call can not do a nonlocal goto or throw.

This results in flow computing an incorrect cfg and deleting code that
it believes is unreachable, but is actually reachable via the nonlocal
goto in the a nested function.

execute/920428-2.c shows this problem.  I've tested it on ia32, ppc & the PA
all with the same basic results.

The mis-compiled function in question:
s(i)
{
  if(i>0)
    {
      __label__ l1;

      int f(int i)
        {
          if(i==2)
            goto l1;
          return 0;
        }

     return f(i);
    l1:;
    }
  return 1;
}

We set TREE_NOTHROW on the function decl for "f".  So when "s" calls "f"
we do not think that the call could do a non-local goto to nonlocal goto
receiver for label "l1" within "s".

So we delete the nonlocal goto receiver, which ultimately results in either
a core dump, an incorrect return value or other nasties.

I'm not entirely sure how you expected this stuff to work for nonlocal
gotos, so I haven't tried to fix it myself.  I would appreciate it if you
would look into this problem.

Thanks,
jeff