If two definitions write to independent subregisters then they can be
put in any order. LiveIntervalAnalysis::handleMove() did not support
this previously because it looks like moving a definition of a vreg past
This is a modified version of a patch proposed (two years ago) by
Vincent Lejeune! This version does not touch the read-undef flags and is
extended for the case of moving a subregister def behind all uses - this
can happen for subregister defs that are completely unused.