Problem 1: There is a typo which reassigns `BlockWorkList` to
`EHPadWorkList` on attempt to remove `RemBB` from work lists.

Problem 2: `Chain->UnscheduledPredecessors == 0` is an incorrect way to
check whether `RemBB` is enqueued or not. The root cause is a postponed
deletion of `WorkList` from already scheduled blocks in
`selectBestCandidateBlock`. Bug happens in the following scenario:

* `FunctionChain` is being processed with non-zero
`UnscheduledPredecessors`
* Block `B'` is added to the `BlockWorkList`
* Block `B'` is chosen as the best successor (`selectBestSuccessor`) for
some another block and added into `Chain`
* Block `B'` is removed by tail duplicator.

`RemovalCallback` erroneously won't erase `B'` from `BlockWorkList`,
because `UnscheduledPredecessors` value of `FunctionChain` is not zero
(and it is allowed to be non-zero).

Proposed solution is to always cleanup worklists on block deletion by
tail duplicator.

MachineBlockPlacement has quadratic runtime in the number of
predecessors: in some situation, for an edge, all predecessors of the
successor are considered.

Limit the number of considered predecessors to bound compile time for
large functions.

Pull Request: https://github.com/llvm/llvm-project/pull/142584

https://github.com/llvm/llvm-project/pull/109711 disables
`buildCFGChains()` when `-apply-ext-tsp-for-size` is used to improve
codesize. Tail merging can change the layout and normally requires
`buildCFGChains()` to be called again, but we want to prevent this when
optimizing for codesize. We saw slight size improvement on large
binaries with this change. If `-apply-ext-tsp-for-size` is not used,
this should be a NFC.