[DebugInfo][3/N] Instruction referencing tests: add extra instr-ref mode lines to several tests

[jmorse]

Summary

In these test updates for instruction referencing, I've added specific instruction-referencing modes, and kept the DBG_VALUE-based variable location check lines. This is because argument handling is really fiddly, and I figured it's worth duplicating the testing to ensure it's definitely correct. There's also (dbg-value-superreg-copy2.mir) a test for where variable locations go when virtual registers are coalesced together. I don't think there's an instruction referencing specific test for this, so figured that's worth duplicating too.

[Orlando]

LGTM