Commits on Oct 27, 2023

1. [InstCombine] Simplify and/or of icmp eq with op replacement …

   and/or in logical (select) form benefit from generic simplifications
   via simplifyWithOpReplaced(). However, the corresponding fold for
   plain and/or currently does not exist.
   
   Similar to selects, there are two general cases for this fold
   (illustrated with `and`, but there are `or` conjugates).
   
   The basic case is something like `(a == b) & c`, where the
   replacement of a with b or b with a inside c allows it to fold
   to true or false. Then the whole operation will fold to either
   false or `a == b`.
   
   The second case is something like `(a != b) & c`, where the
   replacement inside c allows it to fold to false. In that case,
   the operand can be replaced with c, because in the case where
   a == b (and thus the icmp is false), c itself will already be
   false.
   
   As the test diffs show, this catches quite a lot of patterns in
   existing test coverage. I believe this also obsoletes quite a few
   existing special-case and/or of icmp special case folds we have,
   but I haven't removed anything as part of this patch in the
   interest of risk mitigation.

   

   nikic committed Oct 27, 2023
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