Stop depending on the LiveIntervalUnions in RegAllocBase, they are about
to be removed.

The changes are mostly replacing register alias iterators with regunit
iterators, and querying LiveRegMatrix instrad of RegAllocBase.

InterferenceCache is converted to work with per-regunit
LiveIntervalUnions, and it checks fixed regunit interference separately,
using the fixed live intervals provided by LiveIntervalAnalysis.

The local splitting helper calcGapWeights() is also considering fixed
regunit interference which is kept on the side now.

llvm-svn: 158867