[ScopInfo] Consider only write-free dereferencable loads as invariant

When we introduced in r297375 support for hoisting loads that are known
to be dereferencable without any conditional guard, we forgot to keep the check
to verify that no other write into the very same location exists. This
change ensures now that dereferencable loads are allowed to access everything,
but can only be hoisted in case no conflicting write exists.

This resolves llvm.org/PR32778

Reported-by: Huihui Zhang <huihuiz@codeaurora.org>
llvm-svn: 301582