Reverse a loop that is counting up to a maximum to

Reverse a loop that is counting up to a maximum to
count down to 0 instead, under very restricted
circumstances.  Adjust 4 testcases in which this
optimization fires.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71439 91177308-0d34-0410-b5e6-96231b3b80d8