Teach IVUsers to stop at non-affine expressions unless they are both

Teach IVUsers to stop at non-affine expressions unless they are both
outside the loop and reducible.

This more completely hides them from LSR, which isn't usually able to
do anything meaningful with non-affine expressions anyway, and this
consequently hides them from SCEVExpander, which is acutely unprepared
for non-affine expressions.

Replace test/CodeGen/X86/lsr-nonaffine.ll with a new test that tests
the new behavior.

This works around the bug in PR10117 / rdar://problem/9633149, and is
generally an improvement besides.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134268 91177308-0d34-0410-b5e6-96231b3b80d8