NFC: Add a test to LV showing that reduction is not possible when reduction...

NFC: Add a test to LV showing that reduction is not possible when reduction var is reset in the loop

Added a test case to reduction showing where it's illegal to identify
vectorize a loop.
Resetting the reduction var during loop iterations disallows us from
widening the dependency cycle to VF, thereby making it illegal to
vectorize the loop.

llvm-svn: 339605