[EH] Make doEndThrowingInst simpler (NFC) (#4267)
[EH] Make doEndThrowingInst simpler (NFC) (#4267)

The current code the innermost (`i`th) case specially first and handles
`i-1`th `try` in each loop iteration. This puts the `i`th case in the
loop and each iteration handles `i`th `try`, which is simpler. Then we
don't need to check `throwingInstsStack.empty()` in the beginning
because the `for` loop wouldn't be entered if it's empty anyway.
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