Conflict between Stack and Reg uses of the same vreg

Two uses of the same vreg on the same instruction causes an assertion failure if one of them is `Stack` and the other is `Reg` or `FixedReg` since it results in a minimal bundle that cannot be split.

We already handle the case of multiple conflicting `FixedReg` constraints, should we extend this to also cover the case of `Stack`/`Reg` conflicts?