[DSE] Cache liveOnEntry as clobbering access

This builds on @fhahn's D112313, and caches the liveOnEntry node as a optimized access. D112313 tied to only cache a known clobber. This change adds caching the fact that no clobber exists. It still does not cache may-clobber results.

Differential Revision: https://reviews.llvm.org/D120842