[WinEH] Replace more lpad value uses with undef

We were asserting on code like this:
  extern "C" unsigned long _exception_code();
  void might_crash(unsigned long);
  void foo() {
    __try {
      might_crash(0);
    } __except(1) {
      might_crash(_exception_code());
    }
  }

Gtest and many other libraries get the exception code from the __except
block. What's supposed to happen here is that EAX is live into the
__except block, and it contains the exception code. Eventually we'll
represent that as a use of the landingpad ehptr value, but for now we
can replace it with undef.

llvm-svn: 235649