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
db1176ce66cf276b45ac3b90ea6b5ce62de56be7,
relanding commit
598833c987593ce192fa827f162cae8c867c9d43.