[REG2.067a] Issue 13379 - Latest dmd fails with recursive template expansion in std.regex

https://issues.dlang.org/show_bug.cgi?id=13379
When an instance X [1] instantiates Y, then Y instantiates X [2], `inst` == X [1] and this == X [2].
The the old code had been breaking the assumption that `inst` does not appear in the chain of `inst->tinst`.