[llvm] [NFC][LoopVectorize] Make replaceVPBBWithIRVPBB more efficient (PR #111514)

[Paul Walker via llvm-commits]

https://github.com/paulwalker-arm commented:

I don't know enough to know if maintaining the block ordering is what we need/want from a design point of view (it seems kind of fragile to me) but I've added some comments assuming the efficiency gains are worth it.

https://github.com/llvm/llvm-project/pull/111514