This optimization handles unconditional `cond_fail` instructions, i.e. `cond_fail`s with a non-zero `integer_literal` operand.
It cuts off the control flow after such a `cond_fail` by inserting an `unreachable` instruction.
However, this optimization cannot be done as instruction simplification, because it can leave OSSA lifetimes uncompleted.
Other simplification may depend on complete lifetimes.
Similar for constant folding failing casts: we also cannot insert an `unreachable` there.
Instead, do this optimization a new function pass (which can do lifetime completion).
Fixes a SIL verification error
rdar://173728487
Erik Eckstein