When RemoveCopyByCommutingDef is creating additional identity copies, just use

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115385 91177308-0d34-0410-b5e6-96231b3b80d8