[aa-eval] Enhance the comments to better describe the overview of why
this pass exists.

This is based on feedback received when moving this comment from the source
file to a new header file.

Differential Revision: http://reviews.llvm.org/D17476

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262769 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed