[compiler] Avoid creating redundant register moves

They will later be detected as redundant again, but we can save memory
and performance by just not adding them in the first place.

R=mvstanton@chromium.org

Bug: v8:8423
Change-Id: I11d88642333681612e2f8f4eaee7ba700cbf64d5
Reviewed-on: https://chromium-review.googlesource.com/c/1390132Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58479}

src/compiler/backend/instruction.h

@@ -705,9 +705,7 @@ class V8_EXPORT_PRIVATE ParallelMove final
     : public NON_EXPORTED_BASE(ZoneVector<MoveOperands*>),
       public NON_EXPORTED_BASE(ZoneObject) {
  public:
-  explicit ParallelMove(Zone* zone) : ZoneVector<MoveOperands*>(zone) {
-    reserve(4);
-  }
+  explicit ParallelMove(Zone* zone) : ZoneVector<MoveOperands*>(zone) {}
 
   MoveOperands* AddMove(const InstructionOperand& from,
                         const InstructionOperand& to) {
@@ -718,7 +716,9 @@ class V8_EXPORT_PRIVATE ParallelMove final
   MoveOperands* AddMove(const InstructionOperand& from,
                         const InstructionOperand& to,
                         Zone* operand_allocation_zone) {
+    if (from.EqualsCanonicalized(to)) return nullptr;
     MoveOperands* move = new (operand_allocation_zone) MoveOperands(from, to);
+    if (empty()) reserve(4);
     push_back(move);
     return move;
   }

src/compiler/backend/register-allocator.cc

@@ -1795,6 +1795,7 @@ void ConstraintBuilder::MeetConstraintsBefore(int instr_index) {
         UnallocatedOperand(*cur_input, second_output->virtual_register());
     MoveOperands* gap_move = data()->AddGapMove(instr_index, Instruction::END,
                                                 input_copy, *cur_input);
+    DCHECK_NOT_NULL(gap_move);
     if (code()->IsReference(input_vreg) && !code()->IsReference(output_vreg)) {
       if (second->HasReferenceMap()) {
         RegisterAllocationData::DelayedReference delayed_reference = {