Keep track of the last place a live virtreg was used.

Keep track of the last place a live virtreg was used.
This allows us to add accurate kill markers, something the scavenger likes.
Add some more tests from ARM that needed this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103521 91177308-0d34-0410-b5e6-96231b3b80d8