[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

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1237,6 +1237,9 @@ struct DSEState {
       // Reached TOP.
       if (MSSA.isLiveOnEntryDef(Current)) {
         LLVM_DEBUG(dbgs() << "   ...  found LiveOnEntryDef\n");
+        if (CanOptimize && Current != KillingDef->getDefiningAccess())
+          // The first clobbering def is... none.
+          KillingDef->setOptimized(Current);
         return None;
       }