[turbofan] Add backend support for signed loads.

test/cctest/compiler/call-tester.h

Index: test/cctest/compiler/call-tester.h
diff --git a/test/cctest/compiler/call-tester.h b/test/cctest/compiler/call-tester.h
index 27c6044e69527dcafd5d2b57befd75a8a8c21d32..1dbed16372f466ec5a53de82d835cbab07f5f0ea 100644
--- a/test/cctest/compiler/call-tester.h
+++ b/test/cctest/compiler/call-tester.h
@@ -86,12 +86,24 @@ struct ReturnValueTraits<int16_t> {
 };
 
 template <>
+struct ReturnValueTraits<uint16_t> {
+  static uint16_t Cast(uintptr_t r) { return static_cast<uint16_t>(r); }
+  static MachineType Representation() { return kMachUint16; }
+};
+
+template <>
 struct ReturnValueTraits<int8_t> {
   static int8_t Cast(uintptr_t r) { return static_cast<int8_t>(r); }
   static MachineType Representation() { return kMachInt8; }
 };
 
 template <>
+struct ReturnValueTraits<uint8_t> {
+  static uint8_t Cast(uintptr_t r) { return static_cast<uint8_t>(r); }
+  static MachineType Representation() { return kMachUint8; }
+};
+
+template <>
 struct ReturnValueTraits<double> {
   static double Cast(uintptr_t r) {
     UNREACHABLE();