SelectionDAG: Fix bug in ClusterNeighboringLoads

Summary:
The method attempts to find loads that can be legally clustered by
looking for loads consuming the same chain glue token.

However, the old code looks at _all_ users of values produced by the
chain node -- including uses of the loaded/returned value of volatile
loads or atomics. This could lead to circular dependencies which then
failed during scheduling.

With this change, we filter out users by getResNo, i.e. by which
SDValue value they use, to ensure that we only look at users of the
chain glue token.

This appears to be a rather old bug, which is perhaps surprising.
However, the test case is actually quite fragile (i.e., it is hidden
by fairly small changes), and the test _must_ use volatile loads for
the bug to manifest.

Reviewers: arsenm, bogner, craig.topper, foad

Subscribers: MatzeB, jvesely, wdng, hiraditya, javed.absar, jfb, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74253