[ShrinkWrapping] Give up on irreducible CFGs.

[ShrinkWrapping] Give up on irreducible CFGs.
We need to know whether or not a given basic block is in a loop for the analysis
to be correct.
Loop information may be incomplete on irreducible CFGs, therefore we may
generate incorrect code if we use it in those situations.

This fixes PR25988.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257012 91177308-0d34-0410-b5e6-96231b3b80d8