[llvm] [SimplifyCFG] Avoid scanning functions multiple times in `removeUnreachableBlocks` (PR #213416)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> IMHO, removeUnreachableBlocks should not look at instructions at all.

We have EliminateUnreachableBlocks. But it seems to lack maintenance (I mean, the compile-time may get worse), so I'm hesitant to use it.

https://github.com/llvm/llvm-project/pull/213416