aco: rework lower_to_cssa()

aco: rework lower_to_cssa()

This patch changes lower_to_cssa to be much more conservative
about assumptions which phi operands might interfere.
Previously, this pass wasn't exhaustive and could miss some corner cases.

v2: remove optimizations to find better insertion points as it's hard
to guarantee that they are always correct and have overall no benefit.

Fixes: 0b8216b2cdbcaccfd2bd1a65be6b8ac5654e3067 ('aco: Lower to CSSA')

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3385>