Okay, bear with me here...

If you have a setjmp/longjmp situation, it's possible for stack slot coloring to
reuse a stack slot before it's really dead. For instance, if we have something
like this:

1:  y = g;
    x = sigsetjmp(env, 0);
    switch (x) {
    case 1:
      /* ... */
      goto run;  
    case 0:
  run:
      do_run(); /* marked as "no return" */
      break;
    case 3:
      if (...) {
        /* ... */
        goto run;
      }
      /* ... */
      break;
    }

2:  g = y;

"y" may be put onto the stack, so the expression "g = y" is relying upon the
fact that the stack slot containing "y" isn't modified between (1) and (2). But
it can be, because of the "no return" calls in there. A longjmp might come back
with 3, modify the stack slot, and then go to case 0. And it's perfectly
acceptable to reuse the stack slot there because there's no CFG flow from case 3
to (2).

The fix is to disable certain optimizations in these situations. Ideally, we'd
disable them for all "returns twice" functions. But we don't support that
attribute. Check for "setjmp" and "sigsetjmp" instead.

llvm-svn: 104640