[llvm] [Mips][BranchFolding] Enable safe tail merging (PR #225382)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

isLegalToSplitMBBAt seems like it's a hack. The R600 implementation certainly looks like a bug workaround, as does this implementation. I'm less sure what's going on with the Thumb2 version 

https://github.com/llvm/llvm-project/pull/225382