[turbofan] Add basic support for calling to (a subset of) C functions.

src/compiler/code-generator-impl.h

Index: src/compiler/code-generator-impl.h
diff --git a/src/compiler/code-generator-impl.h b/src/compiler/code-generator-impl.h
index 1cab854ce8f2759d194c9bfa584db6520075536e..f0762e9bbec2e0657c6e8372e643c0eed4db47d0 100644
--- a/src/compiler/code-generator-impl.h
+++ b/src/compiler/code-generator-impl.h
@@ -57,6 +57,10 @@ class InstructionOperandConverter {
     return static_cast<uint8_t>(InputInt32(index) & 0x3F);
   }
 
+  ExternalReference InputExternalReference(size_t index) {
+    return ToExternalReference(instr_->InputAt(index));
+  }
+
   Handle<HeapObject> InputHeapObject(size_t index) {
     return ToHeapObject(instr_->InputAt(index));
   }
@@ -108,6 +112,10 @@ class InstructionOperandConverter {
 
   double ToDouble(InstructionOperand* op) { return ToConstant(op).ToFloat64(); }
 
+  ExternalReference ToExternalReference(InstructionOperand* op) {
+    return ToConstant(op).ToExternalReference();
+  }
+
   Handle<HeapObject> ToHeapObject(InstructionOperand* op) {
     return ToConstant(op).ToHeapObject();
   }