Change Artifact.equals() to early-out if hashcodes differ.

java.util.HashMap.get() has an optimization where it compares the
hashcode to a cached value before calling equals() on the candidate key.
CompactHashSet/Map keep a table of cached hashcodes.
But RegularImmutableMap only has a comment saying it assumes the objects in
the collection will optimize equals() with == and hashcode as appropriate!

Simplify use of Objects.equal(); none of the fields are nullable.

Take getClass() out of hashcode computation.  I think two different
Artifact subclasses for the same path is unusual in normal operation,
and I may try to exploit the pass-through hashcode in ActionInputMap.

RELNOTES: None.
PiperOrigin-RevId: 210117899

src/main/java/com/google/devtools/build/lib/actions/Artifact.java

diff --git a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
index fb4f1ed..db97a74 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Artifact.java
@@ -56,7 +56,6 @@
 import java.util.Comparator;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import javax.annotation.Nullable;
 
 /**
@@ -291,7 +290,7 @@
     // The ArtifactOwner is not part of this computation because it is very rare that two Artifacts
     // have the same execPath and different owners, so a collision is fine there. If this is
     // changed, OwnerlessArtifactWrapper must also be changed.
-    this.hashCode = execPath.hashCode() + this.getClass().hashCode() * 13;
+    this.hashCode = execPath.hashCode();
     this.root = root;
     this.execPath = execPath;
     this.rootRelativePath = rootRelativePath;
@@ -711,6 +710,9 @@
   @SuppressWarnings("EqualsGetClass") // Distinct classes of Artifact are never equal.
   @Override
   public boolean equals(Object other) {
+    if (this == other) {
+      return true;
+    }
     if (!(other instanceof Artifact)) {
       return false;
     }
@@ -718,11 +720,11 @@
       return false;
     }
     Artifact that = (Artifact) other;
-    return equalsWithoutOwner(that) && owner.equals(that.getArtifactOwner());
+    return equalsWithoutOwner(that) && owner.equals(that.owner);
   }
 
   public boolean equalsWithoutOwner(Artifact other) {
-    return Objects.equals(this.execPath, other.execPath) && Objects.equals(this.root, other.root);
+    return hashCode == other.hashCode && execPath.equals(other.execPath) && root.equals(other.root);
   }
 
   @Override