[SCEV] Scale back the test added in r294181 as it goes quadratic in

SCEV.

This test was immediately the slowest test in 'check-llvm' even in an
optimized build and was driving up the total test time by 50% for me.

Sanjoy has filed a PR about the quadratic behavior in SCEV but it is
also concerning that the test still passes given that r294181 added
a threshold at 32 to SCEV. I've followed up on the original patch to
figure out how this test should work long-term, but for now I want to
get check-llvm to be fast again.

llvm-svn: 294241