[turbofan] Allow OptionalOperator to return a placeholder.

src/compiler/wasm-compiler.cc

Index: src/compiler/wasm-compiler.cc
diff --git a/src/compiler/wasm-compiler.cc b/src/compiler/wasm-compiler.cc
index dfd05d4c1cc8156c05a74476c6da68a9c307eee2..e7508c26ddffe32340289b2ed64342fb4d809590 100644
--- a/src/compiler/wasm-compiler.cc
+++ b/src/compiler/wasm-compiler.cc
@@ -823,11 +823,12 @@ Node* WasmGraphBuilder::Unop(wasm::WasmOpcode opcode, Node* input,
       op = m->Word64Clz();
       break;
     case wasm::kExprI64Ctz: {
-      if (m->Word64Ctz().IsSupported()) {
-        op = m->Word64Ctz().op();
+      OptionalOperator ctz64 = m->Word64Ctz();
+      if (ctz64.IsSupported()) {
+        op = ctz64.op();
         break;
       } else if (m->Is32() && m->Word32Ctz().IsSupported()) {
-        op = m->Word64CtzPlaceholder();
+        op = ctz64.placeholder();
         break;
       } else if (m->Word64ReverseBits().IsSupported()) {
         Node* reversed = graph()->NewNode(m->Word64ReverseBits().op(), input);
@@ -838,10 +839,11 @@ Node* WasmGraphBuilder::Unop(wasm::WasmOpcode opcode, Node* input,
       }
     }
     case wasm::kExprI64Popcnt: {
-      if (m->Word64Popcnt().IsSupported()) {
-        op = m->Word64Popcnt().op();
+      OptionalOperator popcnt64 = m->Word64Popcnt();
+      if (popcnt64.IsSupported()) {
+        op = popcnt64.op();
       } else if (m->Is32() && m->Word32Popcnt().IsSupported()) {
-        op = m->Word64PopcntPlaceholder();
+        op = popcnt64.placeholder();
       } else {
         return BuildI64Popcnt(input);
       }