[ShrinkWrapping] Fix an infinite loop while looking for restore point.

[ShrinkWrapping] Fix an infinite loop while looking for restore point.
This may happen when the input program itself contains an infinite loop with no
exit block. In that case, we would fail to find a block post-dominating the loop
such that this block is outside of the loop.

This fixes PR24823.
Working on reducing the test case.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247710 91177308-0d34-0410-b5e6-96231b3b80d8