C++ x64 binary decompilation error (simple sum method)

Retdec version: `v4.0-438-gaecb4d08`
Platform: `Ubuntu x64`
Binary: `x64 ELF`
Compiler: `GCC 9.4.0`

Consider simple C++ program:
```
#include

class A {
public:
int a;
int b;
A(int aparam, int bparam) {a = aparam; b=bparam;}
int sum(){return a + b;}
};

int main() {
A a1(1, -1);
printf("%d\n", a1.sum());
}
```
Method `A::sum` compiles with gcc in:
```
; function: _ZN1A3sumEv at 0x1212 -- 0x122f
0x1212: f3 0f 1e fa endbr64
0x1216: 55 push rbp
0x1217: 48 89 e5 mov rbp, rsp
0x121a: 48 89 7d f8 mov qword ptr [rbp - 8], rdi
0x121e: 48 8b 45 f8 mov rax, qword ptr [rbp - 8]
0x1222: 8b 10 mov edx, dword ptr [rax] ; A::a
0x1224: 48 8b 45 f8 mov rax, qword ptr [rbp - 8]
0x1228: 8b 40 04 mov eax, dword ptr [rax + 4] ; A::b
0x122b: 01 d0 add eax, edx ; a + b
0x122d: 5d pop rbp
0x122e: c3 ret
```
And lifts with retdec in:
```
define i64 @_ZN1A3sumEv(i64* %result) local_unnamed_addr {
dec_label_pc_1212:
%0 = alloca i64
%1 = load i64, i64* %0 ; int64_t local
%2 = ptrtoint i64* %result to i64
%3 = trunc i64 %1 to i32 ; (int32_t) local
%4 = add i64 %2, 4, !insn.addr !38
%5 = inttoptr i64 %4 to i32*, !insn.addr !38
%6 = load i32, i32* %5, align 4, !insn.addr !38
%7 = add i32 %6, %3, !insn.addr !39 ; A::b + (int32_t) local
%8 = zext i32 %7 to i64, !insn.addr !39
ret i64 %8, !insn.addr !40
}
```
This function is then decompiled to:
```
// Address range: 0x1212 - 0x122f
// Demangled: A::sum()
int64_t _ZN1A3sumEv(int64_t * result) {
int32_t v1 = *(int32_t *)((int64_t)result + 4); // 0x1228
int64_t v2; // 0x1212
return v1 + (int32_t)v2;
}
```
which is obviously wrong.
As you can see, the error originates from lifted IR, where instructions which dereference the pointer `result+0h` just not generated.

Reproduce the issue with the provided C++ program and inspect the lifted IR for A::sum at 0x1212–0x122f. Trace why the dereference at result+0 is missing; done when both fields are represented and the decompiled method returns the correct sum.

Written by the indexing model from the issue text.