[SEH] Ensure that empty __except blocks have their own BB

The 32-bit lowering assumed that WinEHPrepare had this invariant.
WinEHPrepare did it for C++, but not SEH. The result was that we would
insert calls to llvm.x86.seh.restoreframe in normal basic blocks, which
corrupted the frame pointer.

llvm-svn: 241699