[test] Add reduction sequence test case [NFC]

This test case is a mini performance test case that shows the time needed for a
couple of simple reductions. It takes today about 325ms on my machine to run
this test case through 'opt' with scop construction and reduction detection. It
can be used as mini-proxy for further tuning of the reduction code.

Generally we do not commit performance test cases, but as this is very
small and also very fast it seems OK to keep it in the lit test suite.

This test case will also help to verify that future changes to the reduction
code will not affect the ordering of the reduction sets and will consequently
not cause spurious performance changes that only result from reordering of
dependences in the reduction set.

llvm-svn: 295549