IRCE: generalize to handle loops with decreasing induction variables.

IRCE: generalize to handle loops with decreasing induction variables.

IRCE can now split the iteration space for loops like:

   for (i = n; i >= 0; i--)
     a[i + k] = 42; // bounds check on access

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230618 91177308-0d34-0410-b5e6-96231b3b80d8