r600/sfn: be more conservative with channel use in multi-slot ops

The current approach to check the bank swizzle doesn't allow to re-evaluate bank-swizzle for instructions that where emitted earlier, so we might end up with impossile constellations when we allow three uses of the same channel

Fixes: edabd5cd r600/sfn: check used channels when evaluating allowed mask

Closes: #8350 (closed)