Reland [SimplifyCFG] `markAliveBlocks()`: recognize that normal dest of...

Reland [SimplifyCFG] `markAliveBlocks()`: recognize that normal dest of `invoke`d `noreturn` function is `unreachable`

As per LangRef's definition of `noreturn` attribute:
```
noreturn
  This function attribute indicates that the function never returns
  normally, hence through a return instruction.
  This produces undefined behavior at runtime if the function
  ever does dynamically return. nnotated functions may still
  raise an exception, i.a., nounwind is not implied.
```

So if we `invoke` a `noreturn` function, and the normal destination
of an invoke is not an `unreachable`, point it at the new `unreachable`
block.

The change/fix from the original commit is that we now actually create
the new block, and don't just repurpose the original block,
because said normal destination block could have other users.

This reverts commit db1176ce,
relanding commit 598833c9.