platin: propagate visualization errors on request

Useful when doing batch testing.
platin: minor improvements for visualizer
platin: refactor conflict-free region formation
platin: fix scopegraph consistency check if there are empty blocks
platin: fix VCFG construction for bundled delay slots
platin: fixed VCFG for non-delayed branches
platin: assert that ILP encounters known operator

Only "less-equal" and "equal" are supported throughout. (Even though the solver
seems to know about "greater-equal", the rest of platin does not necessarily do
so.)