StoreInfos is sorted by slice offset, not program order. Anchoring the
IRBuilder at StoreInfos.back() could emit shufflevectors before SSA
values defined later in the same block (invalid IR).
Insert merged shuffles immediately before TheLoad when the load shares
the store block. When the load is elsewhere, insert before the store
block terminator so the merge runs after every store + any trailing
instructions in that block.
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