AMDGPU: Update SlotIndexes independently of LiveIntervals

Apparently StackColoring depends on SlotIndexes, but not
LiveIntervals. If regalloc fast were manually requested, LiveIntervals
would be dropped before SILowerSGPRSpills but not SlotIndexes.

SILowerSGPRSpills preserved SlotIndexes, but only through
LiveIntervals. As a result, SILowerSGPRSpills was incorrectly
reporting it preserved SlotIndexes. Start updating these directly,
instead of depending on LiveIntervals also being available.