[VNCoercion] Use ConstantFoldLoadFromConst API (NFCI)

Nowdays we have a generic constant folding API to load a type from
an offset. It should be able to do anything that VNCoercion can do.

This avoids the weird templating between IRBuilder and ConstantFolder
in one function, which is will stop working as the IRBuilderFolder
moves from CreateXYZ to FoldXYZ APIs.

Unfortunately, this doesn't eliminate this pattern from VNCoercion
entirely yet.