[ValueTracking] Add support for `fshl`/`fshr` in `isKnownToBeAPowerOfTwo`

If the funnel shifts are rotates (op0 == op1) then the number of 1s/0s
don't change so we can just look through op0/op1.

Proofs: https://alive2.llvm.org/ce/z/Pja5yu

Differential Revision: https://reviews.llvm.org/D157307