More conservative fix for issue 12900 - Wrong code in IfStatement condition Expression

Improvement for the PR #3662.

1. Using `t1->implicitConvTo(t2) >= MATCHconst` has a known problem  (eg. [issue 5363](https://issues.dlang.org/show_bug.cgi?id=5363)).
2. Both conversions `*&ex to ex` and `&*ex to ex` should be handled in same way.