[DMD] Loop index incorrectly optimised out for -release -O

[Teodor Dutu]

Hi,

I discovered a bug in DMD whereby, during optimisation, the loop 
index variable is removed, despite being used afterwards. I 
reproduced the bug in [this 
issue](https://issues.dlang.org/show_bug.cgi?id=22372).

Using run.dlang.io, I tried to find a regression, but all 
supported DMD versions (2.060 and newer) are printing the same 
incorrect output (`Exception: i = 0; n = 1`). My experiment can 
be found [here](https://run.dlang.io/is/uYfTzl). This seems to be 
a backend issue, as both LDC and LDC-beta are working fine.

However, when analysing the CI outputs from [the 
PR](https://github.com/dlang/dmd/pull/13116) which made me come 
across this bug, I noticed that all 3 of DMD, LDC and GDC are 
failing the same test, which I've also been able to reproduce 
myself:
- LDC: 
https://cirrus-ci.com/task/6291197929979904?logs=test_druntime#L1449
- DMD: 
https://cirrus-ci.com/task/5728247976558592?logs=test_druntime#L1390
- GDC: 
https://cirrus-ci.com/task/4883823046426624?logs=test_druntime#L1411

The failure of this test is caused by the issue I mentioned above 
and the code with which I reproduced the bug is based on it.

I am unsure what to make of this, since the logs seem to 
contradict my experiment with LDC. Has anyone else encountered 
this? How did you proceed?

Thanks,
Teodor