[LPM] Document the new helpers to make it easy to get consistent require
and preserve behavior from loop passes.

Differential Revision: http://reviews.llvm.org/D17443

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261319 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed