We weren't previously keeping track of quite the right information for whether
an `if .. else .. end`'s following block was reachable or not. It should be
reachable if the head is reachable and either the consequent or alternative end
reachable (and therefore fall through to the following block) or do an early
`br_if` to it.
This commit rejiggers `ControlStackFrame::If` to keep track of reachability at
the end of the consequent (we don't need to keep track of it at the end of the
alternative, since that is simply `state.reachable`) and adds Wasm tests for
every reachability situation we can encounter with `if .. else .. end`.
Fixes#1132
bae0257fc3