[turbofan] Create ExplicitOperands to specify operands without virtual registers

test/unittests/compiler/instruction-sequence-unittest.h

Index: test/unittests/compiler/instruction-sequence-unittest.h
diff --git a/test/unittests/compiler/instruction-sequence-unittest.h b/test/unittests/compiler/instruction-sequence-unittest.h
index e5e5139f970295eb81b4c0c56e0383a020b153ef..eb86bd9174985c883234e468a3ce73c995b75143 100644
--- a/test/unittests/compiler/instruction-sequence-unittest.h
+++ b/test/unittests/compiler/instruction-sequence-unittest.h
@@ -36,6 +36,7 @@ class InstructionSequenceTest : public TestWithIsolateAndZone {
     kFixedRegister,
     kSlot,
     kFixedSlot,
+    kExplicit,
     kImmediate,
     kNone,
     kConstant,
@@ -57,6 +58,11 @@ class InstructionSequenceTest : public TestWithIsolateAndZone {
 
   static TestOperand Same() { return TestOperand(kSameAsFirst, VReg()); }
 
+  static TestOperand ExplicitReg(int index) {
+    TestOperandType type = kExplicit;
+    return TestOperand(type, VReg(), index);
+  }
+
   static TestOperand Reg(VReg vreg, int index = kNoValue) {
     TestOperandType type = kRegister;
     if (index != kNoValue) type = kFixedRegister;