[LICM] Don't try to promote in loops where we have no chance to promote. NFC.

We would check whether we have a prehader *or* dedicated exit blocks,
and go into the promotion loop. Then, for each alias set we'd check
if we have a preheader *and* dedicated exit blocks, and bail if not.

Instead, bail immediately if we don't have both.

llvm-svn: 290728