[LoopUnrollPeel] Add case where we should forget the peeled loop from SCEV.

The test case requires the peeled loop to be forgotten after peeling,
even though it does not have a parent. When called via the unroller,
SE->forgetTopmostLoop is also called, so the test case would also pass
without any SCEV invalidation, but peelLoop is exposed as utility
function. Also, in the test case, simplifyLoop will make changes,
removing the loop from SCEV, but it is better to not rely on this
behavior.

Reviewers: sanjoy, mkazantsev

Reviewed By: mkazantsev

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58192

llvm-svn: 354031