Update on "[compiler] Add fallthrough to branch terminal"

Branch terminals didn't have a fallthrough because they correspond to an outer terminal (optional, logical, etc) that has the "real" fallthrough. But understanding how branch terminals correspond to these outer terminals requires knowing the branch fallthrough. For example, `foo?.bar?.baz` creates terminals along the lines of:

```
bb0:
  optional fallthrough=bb4
bb1:
  optional fallthrough=bb3
bb2:
  ...
  branch ... (fallthrough=bb3)

...

bb3:
  ...
  branch ... (fallthrough=bb4)

...

bb4:
  ...
```

Without a fallthrough on `branch` terminals, it's unclear that the optional from bb0 has its branch node in bb3. With the fallthroughs, we can see look for a branch with the same fallthrough as the outer optional terminal to match them up.

[ghstack-poisoned]

.github/workflows/runtime_commit_artifacts.yml

@@ -87,7 +87,7 @@ jobs:
             build/oss-experimental/react-refresh/cjs/react-refresh-babel.development.js
       - name: Insert @headers into eslint plugin and react-refresh
         run: |
-          sed -i -e 's/ LICENSE file in the root directory of this source tree./ LICENSE file in the root directory of this source tree.\n * \n * @noformat\n * @nolint\n * @lightSyntaxTransform\n * @preventMunge\n * @oncall react_core/' \
+          sed -i -e 's/ LICENSE file in the root directory of this source tree./ LICENSE file in the root directory of this source tree.\n *\n * @noformat\n * @nolint\n * @lightSyntaxTransform\n * @preventMunge\n * @oncall react_core/' \
             build/oss-experimental/eslint-plugin-react-hooks/cjs/eslint-plugin-react-hooks.development.js \
             build/oss-experimental/react-refresh/cjs/react-refresh-babel.development.js
       - name: Move relevant files for React in www into compiled