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

[Alexis Engelke via llvm-commits]

aengelke wrote:

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

Or rather: it isn't used (it is called exactly once in the backend), so nobody ever improved it... I didn't realize that this existed (also given that this is in a different file).

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