[llvm] [InstCombine] Fold (X / C) < X and (X >> C) < X into X > 0 (PR #85555)

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Poseydon42 wrote:

Updated proofs (26 in total) can be found [here](https://alive2.llvm.org/ce/z/TDRxuG). I've fixed most of the issues mentioned here, the only two things I'm worried about are:
  1.The current code does not work with vector constants that have different values in each component (e.g. take a look at test case `lshr_by_const_cmp_sle_value`). I believe this is caused by `m_APInt` not matching when a vector constant has different components, but I am unsure about that. Can something be done about this or should I leave this particular scenario unaffected by the optimization?
  2. It appears that `lshr` already has a similar fold implemented, since test case `lshr_by_const_cmp_ule_value` was not affected by the changes that I've made (it folded to `ret i1 true` both before and after the changes to the code). Can someone confirm that this is the case, and if so, should I remove the `lshr` code path from my PR?

https://github.com/llvm/llvm-project/pull/85555