[regalloc] Fix for verifier overwritting final assessments.

This fixes a subtle register allocation verifier bug: depending on how moves are 
optimized, and when having duplicate phis, we may end up overwriting a final
assessment.

Bug: 
Change-Id: I8b7891efbdd075aae0219d60270f405b13d50f40
Reviewed-on: https://chromium-review.googlesource.com/577288Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46941}

src/compiler/register-allocator-verifier.cc

@@ -455,9 +455,14 @@ void RegisterAllocatorVerifier::ValidatePendingAssessment(
       }
     }
   }
-  // If everything checks out, we may make the assessment.
-  current_assessments->map()[op] =
-      new (zone()) FinalAssessment(virtual_register, assessment);
+  // If everything checks out, we may make the assessment, unless the operand
+  // was redefined in this block: it was initially used for the phi, but then
+  // also used as an output operand.
+  if (current_assessments->map()[op] == nullptr ||
+      current_assessments->map()[op]->kind() == Pending) {
+    current_assessments->map()[op] =
+        new (zone()) FinalAssessment(virtual_register, assessment);
+  }
 }
 
 void RegisterAllocatorVerifier::ValidateFinalAssessment(