Don't try to rotate a loop more than once - we never do this anyway.

Summary:
I can't find a case where we can rotate a loop more than once, and it looks
like we never do this. To rotate a loop following conditions should be met:
1) its header should be exiting
2) its latch shouldn't be exiting

But after the first rotation the header becomes the new latch, so this
condition can never be true any longer.

Tested on with an assert on LNT testsuite and make check.

Reviewers: hfinkel, sanjoy

Subscribers: sebpop, sanjoy, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D20181

llvm-svn: 272439