Mono Windows tests cause asserts: condition var->opcode == OP_REGOFFSET not met

A lot of the tests cause an assert in Mono on Windows:

```
  * Assertion at C:\Users\Alexander\Desktop\Dev\runtime\src\mono\mono\mini\method-to-ir.c:12522, condition `var->opcode == OP_REGOFFSET' not met
```

e.g. src\libraries\System.Collections.Concurrent\tests

/cc @lateralusX 