Fix read out of bounds in IRTypeChecker
Fix read out of bounds in IRTypeChecker

Summary: `begin()` vs `std::prev()` is not symmetric to `end()` vs `std::next()`. The latter actually needs a second check.

Reviewed By: ssj933

Differential Revision: D20124691

fbshipit-source-id: e592d68c55a1f937d10500d22c19f76c4317cb50

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