[llvm] [SCEV] Handle backedge-count logic for std::reverse like loops (PR #92560)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Please revise the Alive2 proof so it doesn't assume the loop is guarded by `icmp slt i8 %left_start, %right_start`; the code in the patch doesn't check for that.  (I think your patch works correctly in that case, but it would be nice to have the proof.)

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Please take a bit of time to clean up the comments (capitalize the first word of a sentence, use "//" for block comments).

https://github.com/llvm/llvm-project/pull/92560