[turbofan] Fix more -Wsign-compare warnings.

src/compiler/x87/code-generator-x87.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/code-generator.h"
 
 #include "src/compilation-info.h"
 #include "src/compiler/code-generator-impl.h"
 #include "src/compiler/gap-resolver.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 2438 matching lines @@
 }
 
 void CodeGenerator::AssembleReturn(InstructionOperand* pop) {
   CallDescriptor* descriptor = linkage()->GetIncomingDescriptor();
 
   // Clear the FPU stack only if there is no return value in the stack.
   if (FLAG_debug_code && FLAG_enable_slow_asserts) {
     __ VerifyX87StackDepth(1);
   }
   bool clear_stack = true;
-  for (int i = 0; i < descriptor->ReturnCount(); i++) {
+  for (size_t i = 0; i < descriptor->ReturnCount(); i++) {
     MachineRepresentation rep = descriptor->GetReturnType(i).representation();
     LinkageLocation loc = descriptor->GetReturnLocation(i);
     if (IsFloatingPoint(rep) && loc == LinkageLocation::ForRegister(0)) {
       clear_stack = false;
       break;
     }
   }
   if (clear_stack) __ fstp(0);
 
   const RegList saves = descriptor->CalleeSavedRegisters();
@@ skipping 264 matching lines @@
     int padding_size = last_lazy_deopt_pc_ + space_needed - current_pc;
     __ Nop(padding_size);
   }
 }
 
 #undef __
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8