Delay dominator updation while cloning loop.

Summary:
Dominator updation fails for a loop inserted with a new basicblock.

A block required by DT to set the IDom might not have been cloned yet. This is because there is no predefined ordering of loop blocks (except for the header block which should be the first block in the list).

The patch first creates DT nodes for the cloned blocks and then separately updates the DT in a follow-on loop.

Reviewers: anemet, dberlin

Subscribers: dberlin, llvm-commits

Differential Revision: http://reviews.llvm.org/D20899

llvm-svn: 272479