[DAG] extractShiftForRotate - replace assertion for shift opcode with an early-out

We feed the result from the first extractShiftForRotate call into the second, and that result might no longer be a shift op (usually due to constant folding).

NOTE: We REALLY need to stop creating nodes on the fly inside extractShiftForRotate!

Fixes Issue #57474

(cherry picked from commit eaede4b5)