[llvm] [ValueTracking] Look through intermediate PHIs for conditional recurrences (PR #215913)

[Ömer Sinan Ağacan via llvm-commits]

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osa1 wrote:

The `L` and `R` variables were renamed recently as they're quite confusing. If you rebase the diff will be a bit more clearer.

https://github.com/llvm/llvm-project/pull/215913