Adjust to ilist changes.

Adjust to ilist changes.

Based on the ilist changes avoid allocating an entire Use object for the
end of the Use chain.  This saves 8 bytes of memory for each Value allocated
in the program.  For 176.gcc, this reduces us from 69.5M -> 66.0M, a 5.3%
memory savings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@19925 91177308-0d34-0410-b5e6-96231b3b80d8