[turbofan] Move some register allocation validation earlier.

This is in an effort to narrow down the root cause of the
referenced, very flaky, bug.

Bug: chromium:725559
Change-Id: I8235983fea6f579d228e81177fdcac9ef6115d1f
Reviewed-on: https://chromium-review.googlesource.com/521923Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45668}

src/compiler/pipeline.cc

@@ -2038,6 +2038,14 @@ void PipelineImpl::AllocateRegisters(const RegisterConfiguration* config,
   Run<AssignSpillSlotsPhase>();
 
   Run<CommitAssignmentPhase>();
+
+  // TODO(chromium:725559): remove this check once
+  // we understand the cause of the bug. We keep just the
+  // check at the end of the allocation.
+  if (verifier != nullptr) {
+    verifier->VerifyAssignment();
+  }
+
   Run<PopulateReferenceMapsPhase>();
   Run<ConnectRangesPhase>();
   Run<ResolveControlFlowPhase>();

src/compiler/register-allocator.cc

@@ -3574,6 +3574,7 @@ void OperandAssigner::CommitAssignment() {
     for (LiveRange* range = top_range; range != nullptr;
          range = range->next()) {
       InstructionOperand assigned = range->GetAssignedOperand();
+      DCHECK(!assigned.IsUnallocated());
       range->ConvertUsesToOperand(assigned, spill_operand);
     }