[turbofan] Add backend support for signed loads.

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index 31f31758b6d6c250a1580b5a0d110b27267d75e7..da0135127f71985ef0b1b857b624d0c6e0fc5a61 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -405,12 +405,14 @@ OStream& operator<<(OStream& os, const PointerMap& pm);
 class Instruction : public ZoneObject {
  public:
   size_t OutputCount() const { return OutputCountField::decode(bit_field_); }
-  InstructionOperand* Output() const { return OutputAt(0); }
   InstructionOperand* OutputAt(size_t i) const {
     DCHECK(i < OutputCount());
     return operands_[i];
   }
 
+  bool HasOutput() const { return OutputCount() == 1; }
+  InstructionOperand* Output() const { return OutputAt(0); }
+
   size_t InputCount() const { return InputCountField::decode(bit_field_); }
   InstructionOperand* InputAt(size_t i) const {
     DCHECK(i < InputCount());