[turbofan] fix too aggressive DCHECK in load elimination

Bug: chromium:1021444, chromium:1033196
Change-Id: Ic0468bce60577ad238581f17c4510a23ab63882a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1964390
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65433}

src/compiler/load-elimination.cc

@@ -661,15 +661,13 @@ LoadElimination::FieldInfo const* LoadElimination::AbstractState::LookupField(
     }
     if (!result.has_value()) {
       result = info;
-    } else {
-      // We detected a partially overlapping access here.
-      // We currently don't seem to have such accesses, so this code path is
-      // unreachable, but if we eventually have them, it is safe to return
-      // nullptr and continue the analysis. But store-store elimination is
-      // currently unsafe for such overlapping accesses, so when we remove
-      // this check, we should double-check that store-store elimination can
-      // handle it too.
-      DCHECK_EQ(**result, *info);
+    } else if (**result != *info) {
+      // We detected inconsistent information for a field here.
+      // This can happen when incomplete alias information makes an unrelated
+      // write invalidate part of a field and then we re-combine this partial
+      // information.
+      // This is probably OK, but since it's rare, we better bail out here.
+      return nullptr;
     }
   }
   return *result;