[llvm] [InstCombine] Propagate poison through fshl and fshr intrinsics (PR #172859)

[Sayan Sivakumaran via llvm-commits]

sivakusayan wrote:

First, thanks for the review you two. 

It seems like there was a [similar discussion](https://reviews.llvm.org/D90382) when that fold was first implemented. If there are problems with unconditionally propagating poison from funnel shifts, maybe only propagating poison in the case of rotation would make sense?

> Doesn't that transform already handle this by inserting freeze if necessary?

That [does seem](https://godbolt.org/#z:OYLghAFBqd5QCxAYwPYBMCmBRdBLAF1QCcAaPECAMzwBtMA7AQwFtMQByARg9KtQYEAysib0QXACx8BBAKoBnTAAUAHpwAMvAFYTStJg1C1aANxakl9ZATwDKjdAGFUtAK4sGIAEwBWUo4AMngMmAByHgBGmMQgAGykAA6oCoR2DC7unj7%2Byam2AsGhESzRsQlWmDbpQgRMxASZHl5%2BlpjWBQy19QRF4VEx8ZZ1DU3ZrQojvSH9pYNxAJSWqG7EyOwcAKQAzABCANQuiXiY6PtUxKgs%2BwCaIcAA7ph4YBwK%2B2J4ppje%2B7QhAGtNhoAIJYGihfZ4AAc%2BwAAgo1t4IDD9ps/KpSFDYejfABPLGo3EKBCsAgLNEAdl2wJB%2BzRfmQLESaO2ABEoUyWZgAI7Yhm%2BElkrEaWn04luSKsjkKSX86FY4mklgEMUCknEaV/DX83EE9XK1Wg8V%2BEm0LVm3UYxWmyVq3EkLWOommhC0G2ChDEe1%2BTU7GXtKoEKFcAVcwk4vz6l2%2BEhq4iYYMx72gzaUtm0zN7fYAWQweBop3Ol2uTEiqG%2B%2ByI%2B0wLEI50tIUmmCYZ1QVAbXtp4Jm/IRa22KMjvkxVvxEYNZIpaZpxrDzK1eC5Nb5MaFKpFPsFcv9%2B1lUtRCsnKq3Ot3tB1Mf1Esip7dFvvMdHSrJW8du%2Bdw7NHo1W797L3QMbBDedEgnXFRxjOM5wTJNhxTEE0zZDglloThfF4LwOC0UhUE4QJAgANRzfYAEkACU9xWNZMAZbYeFIAhNBQpYARAXwNH0ThJF4FgJA0TisJwvCOF4BQQE4pjsJQ0g4FgJA0GZOgYnIShFOOehYlQRICAIYg3AYAE%2BDoAgYnEiBImY0hIhCeo8U4BibOYYg8QAeUibQgwc3hFLYQRXIYWh7Ok0gsEiNxgCcMQL280La0MYBxBC/AE2qb5xJCzBVCqNxTNikJTLQkL/kiYg7JcLArL0vA%2BO4XhvmIcslDZeKjH%2BIxmKWKgDGABRCJOB5XMSRhYv4QQRDEdgpBkQRFBUdQQt0Lh9ASkATHMfQ8EicTICWbTOgygBaRImAUJQFH9JsCEUpzMF4CsYmIPAsB2iAlkqap7AgRwxi8ZaghmEoyj0PI0gEX6QZSMGGD6IHBmWj7Om6UZXGaPREZqKZYYGWIEamCG8Z6bG5lx97qPWCRUPQzCrJE/Z9v2CA9IMgEKQgXBCHfbx6IWXgpK0BYlgQFssFiN7SDYjiuI4HjSCE%2B7ODEiTGM6qmOG8GmQpEvnVdIBrUnsSQgA) to be the case. 

https://github.com/llvm/llvm-project/pull/172859