Assertion failure at lib/Backend/BackwardPass.cpp:8667

The debug build crashes due to assertion failure at lib/Backend/BackwardPass.cpp:8667.
```
let var_0 = [`function foo() {}`];
(function (){
let var_1 = 0;
for (let var_2 of var_0) {
while (true) {
let var_3 = var_2[0];
if (var_3.match(/\s/)) {
try {
[x] = [];
} finally {
new var_3();
}
return {};
}
}
}
return var_1;
})();
```

Commit: ca58245
Build Option: None
OS: Ubuntu 18.04.1 LTS
Arch: x86_64

Start by reproducing the debug-build crash with the provided JavaScript snippet on commit ca58245, then inspect the assertion at lib/Backend/BackwardPass.cpp:8667. Trace the backward-pass state leading to the assertion and determine why this input reaches it. Done means the reproduction no longer crashes while preserving the engine's expected behavior.

Written by the indexing model from the issue text.