[Support] Add convertZoneToTimepoints. NFC.

This function has been extracted from the upcoming DeLICM patch
(https://reviews.llvm.org/D24716).

In contrast to computeReachingWrite and computeArrayUnused,
convertZoneToTimepoints implies a format for zones (ranges between timepoints).
Zones at the moment are unique to DeLICM, but convertZoneToTimepoints makes most
sense in conjunction with the previous two functions.

llvm-svn: 294094