[llvm] [VPlan] Extend cse to eliminate redundant widened loads (PR #212543)

[Ramkumar Ramachandra via llvm-commits]

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artagnon wrote:

Could convert all new tests to VPlan tests, and use -vplan-print-after=cse to make it clear that we're testing CSE specifically?

https://github.com/llvm/llvm-project/pull/212543