When RemoveCopyByCommutingDef is creating additional identity copies, just use

LiveInterval::MergeValueNumberInto instead of trying to extend LiveRanges and
getting it wrong.

This fixed PR8249 where a valno with a multi-segment live range was defined by
an identity copy created by RemoveCopyByCommutingDef. Some of the live
segments disappeared.

llvm-svn: 115385