IR: add FnProto instruction

src/codegen.cpp

@@ -2196,6 +2196,7 @@ static LLVMValueRef ir_render_instruction(CodeGen *g, IrExecutable *executable,
         case IrInstructionIdIntType:
         case IrInstructionIdMemberCount:
         case IrInstructionIdAlignOf:
+        case IrInstructionIdFnProto:
             zig_unreachable();
         case IrInstructionIdReturn:
             return ir_render_return(g, executable, (IrInstructionReturn *)instruction);