[DebugInfo] Check both instr-ref and DBG_VALUE modes of sdag tests

In these test updates for instruction referencing, I've added specific
instr-ref RUN lines, and kep thte DBG_VALUE-based variable location check
lines too. This is because argument handling is really fiddly, and I figure
it's worth duplicating the testing to ensure it's definitely correct.

There's also dbg-value-superreg-copy2.mir, a dtest for where varaible
locations go when virtual registers are coalesced together. I don't think
there's an instruction referencing specific test for this, so have
duplicated that to for instruction referencing.

Differential Revision: https://reviews.llvm.org/D114262