Experiments show that looking through phi nodes

Experiments show that looking through phi nodes
and select instructions doesn't buy anything here
except extra complexity: the only difference in
the entire testsuite was that a readonly function
became readnone in MiBench/consumer-typeset.  Add
a comment about this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61478 91177308-0d34-0410-b5e6-96231b3b80d8