[llvm] Forward-declare ICFLoopSafetyInfo (NFC)

LoopUtils.h needs ICFLoopSafetyInfo but relies on a forward
declaration of ICFLoopSafetyInfo in IVDescriptors.h.  This patch adds
a forward declaration right in LoopUtils.h.

While we are at it, this patch removes the one in IVDescriptors.h,
where it is unnecessary.