Don't allocate memory in LiveInterval::join().

Rewrite value numbers directly in the 'Other' LiveInterval which is
moribund anyway. This avoids allocating the OtherAssignments vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175690 91177308-0d34-0410-b5e6-96231b3b80d8