[ShrinkWrap] Fix FindIDom to only have one kind of failure.

FindIDom() can fail in two different ways - it can either return nullptr or the
block itself, depending on the circumstances. Some users of FindIDom() check
one error condition, while others check the other.

Change it to always return nullptr on failure.
This fixes PR26004.

Differential Revision: http://reviews.llvm.org/D15847

llvm-svn: 256955