This kills 2 birds with one stone:
1. It makes sure generated check_exception() calls in the finalizer
don't mis-read the pending exception as caused by their matching
operation.
2. It implicitly ensures that terminated finally blocks (by a return
statement) overwrite any pending exceptions, since they will never
execute the ContinuePendingUnwind operation that restores the
stashed exception.
This additional logic is required in the JIT (as opposed to the
interpreter), since the JIT uses the exception register to store and
check the possibly-exceptional results from each individual operation,
while the interpreter only modifies it when an operation has thrown an
exception.
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