aco: handle loop exit and IF merge phis with break/discard

aco: handle loop exit and IF merge phis with break/discard

ACO considers discards jumps and creates edges in the CFG for them but NIR
does neither of these.

This can be fixed instead by keeping track of whether a side of an IF had
a break/discard, but this doesn't solve the issue with discards affecting
loop exit phis. So this reworks phi handling a bit.

Fixes these tests:
dEQP-VK.graphicsfuzz.disc-and-add-in-func-in-loop
dEQP-VK.graphicsfuzz.loop-call-discard
dEQP-VK.graphicsfuzz.complex-nested-loops-and-call

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>