Make the hasPHIKills flag a computed property.

The VNInfo::HAS_PHI_KILL is only half supported. We precompute it in
LiveIntervalAnalysis, but it isn't properly updated by live range
splitting and functions like shrinkToUses().

It is only used in one place: RegisterCoalescer::removeCopyByCommutingDef().

This patch changes that function to use a new LiveIntervals::hasPHIKill()
function that computes the flag for a given value number.

llvm-svn: 161254