[VPlan] VPlan cost model and legacy cost model disagreed assertion hit

[vzakhari]

reduced.ll.gz

Reproducer: opt --passes=loop-vectorize reduced.ll:

opt: .../llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7066: llvm::VectorizationFactor llvm::LoopVectorizationPlanner::computeBestVF(): Assertion `(BestFactor.Width == LegacyVF.Width || BestPlan.hasEarlyExit() || planContainsAdditionalSimplifications(getPlanFor(BestFactor.Width), CostCtx, OrigLoop, BestFactor.Width) || planContainsAdditionalSimplifications( getPlanFor(LegacyVF.Width), CostCtx, OrigLoop, LegacyVF.Width)) && " VPlan cost model and legacy cost model disagreed"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.

The issue appeared between 3addfd0 and 988ab5a. It affects CPU2017/621.wrf_s.

The only suspect (by just looking through the commits) is #154617. @fhahn can you please take a look?