[arm64] Fix GCC build warnings.

src/compiler/arm64/instruction-selector-arm64.cc

Index: src/compiler/arm64/instruction-selector-arm64.cc
diff --git a/src/compiler/arm64/instruction-selector-arm64.cc b/src/compiler/arm64/instruction-selector-arm64.cc
index 9000888e3043fcd1c0c2d6f7bad359d0f504656c..7114077ff9a86328142000c3e3fc84c2936191ee 100644
--- a/src/compiler/arm64/instruction-selector-arm64.cc
+++ b/src/compiler/arm64/instruction-selector-arm64.cc
@@ -2249,19 +2249,15 @@ void InstructionSelector::VisitAtomicLoad(Node* node) {
   Node* base = node->InputAt(0);
   Node* index = node->InputAt(1);
   ArchOpcode opcode = kArchNop;
-  ImmediateMode immediate_mode = kNoImmediate;
   switch (load_rep.representation()) {
     case MachineRepresentation::kWord8:
       opcode = load_rep.IsSigned() ? kAtomicLoadInt8 : kAtomicLoadUint8;
-      immediate_mode = kLoadStoreImm8;
       break;
     case MachineRepresentation::kWord16:
       opcode = load_rep.IsSigned() ? kAtomicLoadInt16 : kAtomicLoadUint16;
-      immediate_mode = kLoadStoreImm16;
       break;
     case MachineRepresentation::kWord32:
       opcode = kAtomicLoadWord32;
-      immediate_mode = kLoadStoreImm32;
       break;
     default:
       UNREACHABLE();