Converting to 'Conventional SSA Form' ensures correctness w.r.t. spilling of phi nodes.
Previously, it was possible that phi operands have intersecting live-ranges, and thus,
couldn't get spilled to the same spilling slot. For this reason, ACO tried to avoid to
spill phis, even if it was beneficial.
This patch implements a conversion pass which is currently only called if spilling is necessary.
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
0b8216b2cd