[llvm] [SLP] Add store-to-load forwarding conflict cost for widened store chains (PR #199606)

[Milin Bhade via llvm-commits]

mbhade-amd wrote:

> Also, check that multi-chain loops do not over-charge. You'd better to collect conflicting loads in findStoreLoadForwardingConflict and check that each used only once in each check to avoid double costing

Fixed — commit-scoped dedup, only charges once per load across trees. Added tests for both the waiver and a distinct-loads negative control.

https://github.com/llvm/llvm-project/pull/199606