Remove a bunch of old SCEVExpander FIXME's for preserving NoWrap.

The great thing about the SCEVAddRec No-Wrap flag (unlike nsw/nuw) is
that is can be preserved while normalizing (reassociating and
factoring).

The bad thing is that is can't be tranfered back to IR, which is one
of the reasons I don't like the concept of SCEVExpander.

Sorry, I can't think of a direct way to test this, which is why these
were FIXMEs for so long. I just think it's a good time to finally
clean it up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186273 91177308-0d34-0410-b5e6-96231b3b80d8