Fix a bug (which I introduced) in handling of linear regressions on linear version numbers.  This resolves the “unsupported operand type(s) for -: ‘tuple’ and ‘tuple’” server error when rendering linear regressions.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@188194 91177308-0d34-0410-b5e6-96231b3b80d8