This is a common pattern with dyn_cast and similar constructs, when the
PHI no longer depends on the select it can often be turned into a simpler
construct or even get hoisted out of the loop.
PR15340.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175995 91177308-0d34-0410-b5e6-96231b3b80d8