[AMDGPU][Scheduler] Fix compile failure due to const/sort interaction (#175755)
On some configurations sorting `ScoredRemat` objects which contains
const members causes a compile failure due to impossibility of
swapping/moving objects. The problem was introduced in #175050.
This removes const from those fields to address the issue. The design
will soon change anyway to not rely on sorting objects of this type, and
consts were only here for semantic clarity.