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

[Iain Buclaw]

On Saturday, 9 October 2021 at 21:09:19 UTC, Teodor Dutu wrote:
> 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
>

Just to clarify, those are all DMD.  The difference in those 
pipelines is who built DMD.