dillon1000/react

Fix missing declaration invariant for "for" within try/catch

Fixes T180509722. What happened is that the logic in LeaveSSA to find 
declarations within for initializers wasn't working with try/catch because the 
initializer block gets broken up with a maybe-throw after every instruction that 
can throw. These maybe-throws can then get turned into gotos by 
PruneMaybeThrows, so LeaveSSA has to handle both cases. 

The new logic scans from the start of the init block until reaching the end, and 
creates declarations for all StoreLocals. Note that we don't yet support 
maybe-throw in value blocks — that's already a todo — so the change here simply 
allows us to compile farther until reaching that other todo. But i've 
double-checked the HIR and it looks correct for this case, so it should just 
work once we fix that todo. I've also added a comment to help us remember (and 
of course, we'd have to add a snap fixture too)