[PM] Add a comment clarifying what a particular predicate is doing.

This came up as a point of confusion while working on a fundamental
problem with the combination of CGSCC iteration and the inliner.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309662 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed