When a dynamic index of -1 (the kPoisonIndex sentinel) was folded into
the static position of a vector.insert op,
foldDenseElementsAttrDestInsertOp would proceed to call
calculateInsertPosition, which returned -1. The subsequent iterator
arithmetic (allValues.begin() + (-1)) was undefined behaviour, causing
an assertion in DenseElementsAttr::get.
Fix by bailing out early in foldDenseElementsAttrDestInsertOp when any
static position equals kPoisonIndex, consistent with how
InsertChainFullyInitialized already guards this case.
Fixes#188404
Assisted-by: Claude Code
00c6b4dabd