[vm, compiler] Remove some unnecessary loads of null on ARM64.

TEST=ci
Change-Id: Ie3cb4bb919a200d22853184c0c28903efc2e5bd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218783
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

runtime/vm/compiler/backend/flow_graph_compiler_arm64.cc

@@ -360,13 +360,11 @@ void FlowGraphCompiler::EmitPrologue() {
     }
 
     __ Comment("Initialize spill slots");
-    if (num_locals > 1 || (num_locals == 1 && args_desc_slot == -1)) {
-      __ LoadObject(R0, Object::null_object());
-    }
     for (intptr_t i = 0; i < num_locals; ++i) {
       const intptr_t slot_index =
           compiler::target::frame_layout.FrameSlotForVariableIndex(-i);
-      Register value_reg = slot_index == args_desc_slot ? ARGS_DESC_REG : R0;
+      Register value_reg =
+          slot_index == args_desc_slot ? ARGS_DESC_REG : NULL_REG;
       __ StoreToOffset(value_reg, FP, slot_index * kWordSize);
     }
   }

runtime/vm/compiler/backend/il_arm64.cc

@@ -2705,10 +2705,8 @@ static void InlineArrayAllocation(FlowGraphCompiler* compiler,
   // R3: new object end address.
   // R8: iterator which initially points to the start of the variable
   // data area to be initialized.
-  // R6: null
   if (num_elements > 0) {
     const intptr_t array_size = instance_size - sizeof(UntaggedArray);
-    __ LoadObject(R6, Object::null_object());
     __ AddImmediate(R8, AllocateArrayABI::kResultReg,
                     sizeof(UntaggedArray) - kHeapObjectTag);
     if (array_size < (kInlineArraySize * kCompressedWordSize)) {
@@ -2718,7 +2716,7 @@ static void InlineArrayAllocation(FlowGraphCompiler* compiler,
             AllocateArrayABI::kResultReg,
             compiler::Address(R8, current_offset, compiler::Address::Offset,
                               compiler::kObjectBytes),
-            R6);
+            NULL_REG);
         current_offset += kCompressedWordSize;
       }
     } else {
@@ -2730,7 +2728,7 @@ static void InlineArrayAllocation(FlowGraphCompiler* compiler,
           AllocateArrayABI::kResultReg,
           compiler::Address(R8, 0, compiler::Address::Offset,
                             compiler::kObjectBytes),
-          R6);
+          NULL_REG);
       __ AddImmediate(R8, kCompressedWordSize);
       __ b(&init_loop);
       __ Bind(&end_loop);