[dfsan] Propagate origin tracking at load

This is a part of https://reviews.llvm.org/D95835.

One issue is about origin load optimization: see the
comments of useCallbackLoadLabelAndOrigin

@gbalats This change may have some conflicts with your 8bit change. PTAL the change at visitLoad.

Reviewed By: morehouse, gbalats

Differential Revision: https://reviews.llvm.org/D97570