[TargetLowering] Simplify the interface for expandCTPOP/expandCTLZ/expandCTTZ.

There is no need to return a bool and have an SDValue output
parameter. Just return the SDValue and let the caller check if it
is null.

I have another patch to add more callers of these so I thought
I'd clean up the interface first.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112267