[SROA] Add a debugging tool which shuffles the slices sequence prior to
sorting it. This helps uncover latent reliance on the original ordering
which aren't guaranteed to be preserved by std::sort (but often are),
and which are based on the use-def chain orderings which also aren't
(technically) guaranteed.

Only available in C++11 debug builds, and behind a flag to prevent noise
at the moment, but this is generally useful so figured I'd put it in the
tree rather than keeping it out-of-tree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202106 91177308-0d34-0410-b5e6-96231b3b80d8