[InstCombine] narrowRotate - minor refactoring for funnel shift support. NFC.

Prep work for PR35155 - renamed narrowRotate to narrowFunnelShift, rewrote some comments and adjusted code to collect separate shift values, although we bail if they don't match (still only rotations are only actually folded).

I'm trying to match matchFunnelShift as much as possible in case we finally get to merge these one day.