[llvm] [VPlan] CSE ExpandSCEV recipes (PR #208953)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> > Is the stack on other PRs needed to show any improvements with CSE support for VPExpandSCEVRecipe? If not, may be good to unstack, as it may help with the regressions from #209921
> 
> Will check, thanks, but don't we need to remove the dead ExpandSCEVs after CSE'ing them? #208952

Rebased. This patch simply eliminates the need for removeRedundantExpandSCEVs, and produces identical IR, when CSE is in the VPlanTransforms::optimize pipeline. When CSE is not in the VPlanTransforms::optimize pipeline, this change is a no-op, as ExpandSCEVs are not found at the point at which CSE is run, late in the optimization pipeline, and it cannot eliminate the need for removeRedundantExpandSCEVs, making it pointless.

https://github.com/llvm/llvm-project/pull/208953