[IRBuilder] Introduce folder using inst-simplify, use for Or fold.

Alternative to D116817.

This introduces a new value-based folding interface for Or (FoldOr),
which takes 2 values and returns an existing Value or a constant if the
Or can be simplified. Otherwise nullptr is returned. This replaces the
more restrictive CreateOr which takes 2 constants.

This is the used to implement a folder that uses InstructionSimplify.
The logic to simplify `Or` instructions is moved there. Subsequent
patches are going to transition other CreateXXX to the more general
FoldXXX interface.

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D116935