Remove unnecessary IDom check

Summary: This Idom check seems unnecessary. The immediate children of a node on the Dominator Tree should always be the IDom of its immediate children in this case.

Reviewers: hfinkel, majnemer, dberlin

Reviewed By: dberlin

Subscribers: dberlin, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D26954

llvm-svn: 298232