[SelectionDAG] Fix crash for salvaging with indirect debug values (#72645)

This is a follow-up to #68981, and fix for #72630, #72447.

We may end up in SelectionDAG::salvageDebugInfo() with indirect debug
values, and attempting to salvage ADD nodes with non-constant RHS would
lead us to try to turn those indirect debug values variadic, which is
not allowed.

This triggered the following assert in the SDDbgValue constructor:

  Assertion `!(IsVariadic && IsIndirect)' failed.

This also adds a lit test for salvaging when having an indirect debug
value and constant RHS, as there seems like there was no such lit test.
However, I am not sure if the use of the stack_value operation is
correct in that case (which is existing behavior before #68981), but
that at least documents the current behavior.