Fix more -Wsign-compare warnings in heap, mips, base, etc.

src/mips64/simulator-mips64.cc

Index: src/mips64/simulator-mips64.cc
diff --git a/src/mips64/simulator-mips64.cc b/src/mips64/simulator-mips64.cc
index 02387d0f4fae6bb37ab1773b7ffb43d162f1f477..4a8e0076d9d3094cc5524fb50d06f7b79541579d 100644
--- a/src/mips64/simulator-mips64.cc
+++ b/src/mips64/simulator-mips64.cc
@@ -4879,7 +4879,7 @@ void Simulator::Execute() {
     while (program_counter != end_sim_pc) {
       Instruction* instr = reinterpret_cast<Instruction*>(program_counter);
       icount_++;
-      if (icount_ == static_cast<uint64_t>(::v8::internal::FLAG_stop_sim_at)) {
+      if (icount_ == static_cast<int64_t>(::v8::internal::FLAG_stop_sim_at)) {
         MipsDebugger dbg(this);
         dbg.Debug();
       } else {