[turbofan] Turn IrOpcode::Mnemonic() into a table lookup.

src/compiler/opcodes.h

Index: src/compiler/opcodes.h
diff --git a/src/compiler/opcodes.h b/src/compiler/opcodes.h
index ccdec4d215f1ba75c825a752ce82ccdc7edd1289..3f00e6a178668a4d623e6c5855dc4a370b26d3aa 100644
--- a/src/compiler/opcodes.h
+++ b/src/compiler/opcodes.h
@@ -268,18 +268,7 @@ class IrOpcode {
   };
 
   // Returns the mnemonic name of an opcode.
-  static const char* Mnemonic(Value val) {
-    // TODO(turbofan): make this a table lookup.
-    switch (val) {
-#define RETURN_NAME(x) \
-  case k##x:           \
-    return #x;
-      ALL_OP_LIST(RETURN_NAME)
-#undef RETURN_NAME
-      default:
-        return "UnknownOpcode";
-    }
-  }
+  static char const* Mnemonic(Value value);
 
   static bool IsJsOpcode(Value val) {
     switch (val) {