Revert "Revert "Temporarily do not drop volatile stores before unreachable""

This reverts commit 52aeacfb.
There isn't full agreement on a path forward yet, but there is agreement that
this shouldn't land as-is.  See discussion on https://reviews.llvm.org/D105338

Also reverts unreviewed "[clang] Improve `-Wnull-dereference` diag to be more in-line with reality"
This reverts commit f4877c78.

And all the related changes to tests:
This reverts commit 9a015279.
This reverts commit 3f7c9cc2.
This reverts commit 329f8197.
This reverts commit aa9f58cc.
This reverts commit 2df37d5d.
This reverts commit a72a4418.