[NFC] Introduce ICFLoopSafetyInfo

This is an alternative implementation of LoopSafetyInfo that uses the implicit
control flow tracking to give precise answers on queries "whether or not this
block contains throwing instructions". This rules out false-positive answers on
LoopSafetyInfo's queries.

This patch only introduces the new implementation. It is not currently used in
any pass. The enabling patches will go separately, through review.

The plan is to completely replace all uses of LoopSafetyInfo with
ICFLoopSafetyInfo in the future, but to avoid introducing functional problems,
we will do it pass by pass.

llvm-svn: 344601