Undeclared identifier remains undetected until QBE code generation

```
main() {
    a = 42;
}
```
```
vtereshkov@Vasiliy-Acer:~/cbc/examples$ ../cbc -o ./test ./hello_world.b 
qbe:output.qbe:5: invalid type for second operand %.a_0 in storel
combined source:0:0: error: QBE failed to process the intermediate representation
```