`mlir::matchReduction()` accessed `terminatorOp->getOperand(redPos)`
without checking that `redPos` is within the terminator's operand count.
This caused an assertion failure when the region's block-argument count
exceeds the terminator's yield count, e.g. for `linalg.pooling_nhwc_sum`
whose kernel region has three block args but yields only one value.
Add a bounds check before the operand access so the function returns
nullptr gracefully instead of crashing.
Fixes#131437
2a6a62ac33