Fix `FindSingleBitChange` to handle NOT(V) where V is not an Instruction

Was previously buggy to assume that NOT'd Value was always an
instruction. If the NOT'd value is not an Instruction, we should just
return as its either a constant, in which can we will re-run the logic
after constant-folding, or its a type we can't evaluate anyways.

This is a follow up to: `D140939`

Reviewed By: pengfei, RKSimon

Differential Revision: https://reviews.llvm.org/D142339