RegisterCoalescer: Fix stripCopies() picking up main range instead of subregister range

This fixes a problem where stripCopies() would switch to values in the
main liverange when it crossed a copy instruction. However when joining
subranges we need to stay in the respective subregister ranges.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224461 91177308-0d34-0410-b5e6-96231b3b80d8