We can sometimes statically determine that a given load will unconditionally
trap. When this happens, we emit an unconditional trap, and we need to stop
adding new instructions to the block. This commit introduces a `Reachability<T>`
type that is like `Option<T>` but specifically for communicating reachability
and is marked `must_use` to receivers to handle transitions from reachable to
unreachable states.
Additionally, adds handling of reachable -> unreachable state transitions to
some SIMD op translations that weren't checking for it.
Fixes#5455Fixes#5456
276bc6ad2e