Completely rewrite 'correct' eh support. This changes how setjmp insertion

is performed so it is only at most once per function that contains an invoke
instead of once per invoke in the function.  This patch has the following perks:

1. It fixes PR631, which complains about slowness.
2. If fixes PR240, which complains about non-volatile vars being live across
   setjmp/longjmps.
3. It improves (but does not fix) the jmpbuf alignment issue on itanium by not
   forcing the jmpbufs to always be 8-bytes off the alignment of the structure.
4. It speeds up 253.perlbmk from 338s to 13.70s (a 25x improvement!), making us
   now about 4% faster than GCC.

Further improvements are also possible.

llvm-svn: 23477