Do same as JDK-8302652 but for Vector API.

[~jrose] Sketched it like this:

From
int a = 0; for (…) { … a += v.reduceLanes(ADD) … }
to
vector<int> asplit = zeroes(); for (…) { ... asplit = asplit.add(v) … }; int a = asplit.reduceLanes(ADD);


I coded up concreate int-add-reduction example (dot-product).

./java -Xbatch -XX:CompileCommand=printcompilation,Test::* -XX:CompileCommand=exclude,Test::test00 -XX:UseAVX=2 -XX:CompileCommand=print,Test::test11 Test.java > txt1.txt

./java -Xbatch -XX:CompileCommand=printcompilation,Test::* -XX:CompileCommand=exclude,Test::test00 -XX:UseAVX=2 -XX:CompileCommand=print,Test::test12 Test.java > txt1.txt

grepping for "vector_reduction_int", we see that for test11 we have many reductions in the loop, whereas for test12 we only have 2 reductions in the whole compilation.