[ShrinkWrapping] Do not choose restore point inside loops.

The post-dominance property is not sufficient to guarantee that a restore point
inside a loop is safe.
E.g.,
 while(1) {
   Save
   Restore
   if (...)
     break;
   use/def CSRs
 }
All the uses/defs of CSRs are dominated by Save and post-dominated
by Restore. However, the CSRs uses are still reachable after
Restore and before Save are executed.

This fixes PR25824

llvm-svn: 255613