[llvm] [SCEV] Try to re-use pointer LCSSA phis when expanding SCEVs. (PR #147824)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Ping, now that https://github.com/llvm/llvm-project/pull/147214 landed.

This simplifies runtime checks in a number of cases already, and prevents
regressions with https://github.com/llvm/llvm-project/pull/142309, which
turns some phis into single-entry ones, which SCEV will look through
now (and expand the whole AddRec), whereas before it would have to treat
the LCSSA phi as SCEVUnknown.

https://github.com/llvm/llvm-project/pull/147824