Correctly handle multiple DBG_VALUE instructions at the same SlotIndex.

It is possible to have multiple DBG_VALUEs for the same variable:

32L TEST32rr %vreg0<kill>, %vreg0, %EFLAGS<imp-def>; GR32:%vreg0
    DBG_VALUE 2, 0, !"i"
    DBG_VALUE %noreg, %0, !"i"

When that happens, keep the last one instead of the first.

llvm-svn: 136842