[llvm] [LICM][MustExec] Make must-exec logic for IV condition commutative (PR #93150)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

We can still use the information, I think?

In the simplest case, we have a loop with a single exit, that we can prove is finite some other way (mustprogress etc.).  Then we can just report N+1, because the overflow case would be undefined.

The given testcase is a bit more complicated: you have two exits.  But still, we know the loop is finite.  The backedge-taken count is something like `umin(zext(OtherExit), zext(N)+1)`.   But that's equivalent to `trunc(umin(zext(OtherExit), zext(N)+1))`.  And we can simplify that to `umin(OtherExit, umax(N, N+1))`.

Even if we can't prove the loop is finite, recording the information could be useful... see https://github.com/llvm/llvm-project/pull/83052#pullrequestreview-2013190120 .

https://github.com/llvm/llvm-project/pull/93150