[SEH] Deal with users of the old lpad for SEH catch-all blocks

The way we split SEH catch-all blocks can leave some dead EH values
behind at -O0. Try to remove them, and if we fail, replace them all with
undef.

Fixes a crash when removing the old unreachable landingpad which is
still used by extractvalue instructions in the catch-all block.

llvm-svn: 235061