[turbofan] Allow encodable 64-bit constants as immediate for ARM64.

src/compiler/instruction-selector-unittest.h

Index: src/compiler/instruction-selector-unittest.h
diff --git a/src/compiler/instruction-selector-unittest.h b/src/compiler/instruction-selector-unittest.h
index c236853f0761c76a9de2068489f637058c669989..4e12dabbe1898d6a1a54911f86bae5d0a47b69c6 100644
--- a/src/compiler/instruction-selector-unittest.h
+++ b/src/compiler/instruction-selector-unittest.h
@@ -147,6 +147,10 @@ class InstructionSelectorTest : public TestWithContext, public TestWithZone {
       return ToConstant(operand).ToInt32();
     }
 
+    int64_t ToInt64(const InstructionOperand* operand) const {
+      return ToConstant(operand).ToInt64();
+    }
+
     int ToVreg(const InstructionOperand* operand) const {
       if (operand->IsConstant()) return operand->index();
       EXPECT_EQ(InstructionOperand::UNALLOCATED, operand->kind());