LiveIntervalAnalysis: Support moving of subregister defs in handleMove

This is an updated version which fixes a bug that happened with
uses tied to an earlyclobber operand which end at an unusual slotindex.

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

llvm-svn: 260906