[es2015] Precompute the descriptive string for symbols.

src/builtins/mips/builtins-mips.cc

Index: src/builtins/mips/builtins-mips.cc
diff --git a/src/builtins/mips/builtins-mips.cc b/src/builtins/mips/builtins-mips.cc
index 24fe271cb346d02bb131e52d391a5456c5ed8c75..dd19b2383cf0a41caedc90224aecd0944fdc6109 100644
--- a/src/builtins/mips/builtins-mips.cc
+++ b/src/builtins/mips/builtins-mips.cc
@@ -298,12 +298,8 @@ void Builtins::Generate_StringConstructor(MacroAssembler* masm) {
 
   // 3b. Convert symbol in a0 to a string.
   __ bind(&symbol_descriptive_string);
-  {
-    __ Lsa(sp, sp, t0, kPointerSizeLog2);
-    __ Drop(1);
-    __ Push(a0);
-    __ TailCallRuntime(Runtime::kSymbolDescriptiveString);
-  }
+  __ lw(v0, FieldMemOperand(a0, Symbol::kDescriptiveStringOffset));
+  // Fall through.
 
   __ bind(&drop_frame_and_ret);
   {