Improve descriptions of combiner-alias-analysis and combiner-global-alias-analysis

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200087 91177308-0d34-0410-b5e6-96231b3b80d8
Add combiner-aa-only-func (debug only)

This option (which is !NDEBUG only) allows restricting the use of alias
analysis in DAGCombiner to a specific function. This has proved extremely
valuable to isolating bugs related to this feature, and mirrors the
misched-only-func option provided by the new instruction scheduler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200088 91177308-0d34-0410-b5e6-96231b3b80d8