[llvm] [BranchFolding] Fix assertion failure in HoistCommonCodeInSuccs (PR #141028)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

> /me squints -- I think it's merging the load-to-r0 from bb.3 and bb.6 into bb.1, and leaving the second load-to-r0 in bb.6. It also simplifies away the self-loop into being a single-block-self-loop. This seems alright. It should be fine to land, but @OCHyams could you confirm that it produces the same output as before #140063 before landing?

Confirmed that the output is the same before as before #140063. For additional confidence, you can see the splice I've added here is just a copy+paste of the one removed in #140063.

I've improved the test comment.

https://github.com/llvm/llvm-project/pull/141028