[MachineVerifier] Verify that a DBG_VALUE has a debug location

Summary:
Verify that each DBG_VALUE has a debug location. This is required by
LiveDebugValues, and perhaps by other late passes.

There's an exception for tests: lots of tests use a two-operand form of
DBG_VALUE for convenience. There's no reason to prevent that.

This is an extension of D80665, but there's no dependency.

Reviewers: aprantl, jmorse, davide, chrisjackson

Subscribers: hiraditya, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, jrtc27, MaskRay, zzheng, edward-jones, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei, Jim, lenary, s.egerton, pzheng, sameer.abuasal, apazos, luismarques, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80670