LiveIntervalAnalysis: Support moving of subregister defs in handleMove
LiveIntervalAnalysis: Support moving of subregister defs in handleMove

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
another one.

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.

Differential Revision: http://reviews.llvm.org/D9067

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@260565 91177308-0d34-0410-b5e6-96231b3b80d8

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