[LVI] Fix and re-enable at-use reasoning (PR60629)

This fixes the handling of phi nodes in getConstantRangeAtUse()
and re-enables it, reverting the workaround from
c77c186a.

For phi nodes, while we can make use of the edge condition for the
incoming value, we shouldn't look past the phi node to look for
further conditions, because we might be reasoning about values
from two different cycle iterations (which will have the same
SSA value).

To handle this more specifically we would have to detect cycles,
and there doesn't seem to be any motivating case for that at this
point.