Avoid creating a SlotIndex from the end() iterator.

No test case, spotted by inspection.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176705 91177308-0d34-0410-b5e6-96231b3b80d8
Rewrite the physreg part of findLastUseBefore().

To find the last use of a register unit, start from the bottom and scan
upwards until a user is found.

<rdar://problem/13353090>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176706 91177308-0d34-0410-b5e6-96231b3b80d8