Ensure `collectTransitivePredecessors` returns Pred only from the Loop. (#113831)

It's possible that we encounter Irreducible control flow, due to which,
we may find that a few predecessors of BB are not a part of the CurLoop.
Currently we crash in the function for such cases. This patch ensures
that we only return Predecessors that are a part of CurLoop and
gracefully ignore other Predecessors.

For example, consider Irreducible IR of this form:
```
define i64 @baz() {
bb:
  br label %bb1

bb1:                                              ; preds = %bb3, %bb
  br label %bb3

bb2:                                              ; No predecessors!
  br label %bb3

bb3:                                              ; preds = %bb2, %bb1
  %load = load ptr addrspace(1), ptr addrspace(1) null, align 8
  br label %bb1
}
```

This crashes when `collectTransitivePredecessors` is called on the
`%bb1<Header>, %bb3<latch>` loop, because the loop body has a
predecessor `%bb2` which is not a part of the loop.

See https://godbolt.org/z/E9fM1q3cT for the crash

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