Reverse-order reduction loops seem to to be vectorized.

Reproduce with attached file - swap between increment and decrement loop:
java -XX:-TieredCompilation -Xbatch -XX:CompileCommand=compileonly,Test::test -XX:+TraceSuperWord -XX:+TraceNewVectors Test.java

Maybe it is as simple as allowing reductions to go in both directions in "SuperWord::reduction".
Currently, we only seem to allow s1 to be input of s2, and not the other way around.