It turns out that with the current scev organization ReuseOrCreateCast cannot

It turns out that with the current scev organization ReuseOrCreateCast cannot
know where users will be added. Because of this, it cannot use
Builder.GetInsertPoint at all.

This patch
* removes the FIXME about adding the assert.
* adds a comment explaining hy we don't have one.
* removes a broken logic that only works for some callers and is not needed
  since r150884.
* adds an assert to caller that would have caught the bug fixed by r150884.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151015 91177308-0d34-0410-b5e6-96231b3b80d8