Add support for reduction variables that do not start at zero.

Add support for reduction variables that do not start at zero.
This is important for nested-loop reductions such as :

In the innermost loop, the induction variable does not start with zero:

for (i = 0 .. n)
 for (j = 0 .. m)
  sum += ...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166387 91177308-0d34-0410-b5e6-96231b3b80d8