[Support/DepInfo] Introduce IslMaxOperationsGuard and make DepInfo use it. NFC.

IslMaxOperationsGuard defines a scope where ISL may abort operations because if
it takes too many operations. Replace the call to the raw ISL interface by a
use of the guard.

IslMaxOperationsGuard provides a uniform way to define a maximal computation
time for a code region in C++ using RAII.

llvm-svn: 283744