[SelectionDAG] Salvage debug info for non-constant ADDs (2nd try) (#68981)

Teach SelectionDAG::salvageDebugInfo() to salvage debug information for
ADD nodes where the RHS is non-constant.

In the first try, the test case used the MIR output that was produced by
using -stop-before=sparc-isel. Running -start-before=sparc-isel on that
output resulted in the following verifier error with EXPENSIVE_CHECKS:

  "Function has NoVRegs property but there are VReg operands".

In this re-attempt the test case has been rewritten to a .ll test by
extracting the IR from the original MIR file. The test still starts
before sparc-isel.

Co-authored-by: Mikael Holmen <mikael.holmen@ericsson.com>