[DebugInfo][InstrRef] Don't fire assertions if debug-info is faulty

It's inevitable that optimisation passes will fail to update debug-info:
when that happens, it's best if the compiler doesn't crash as a result.
Therefore, downgrade a few assertions / failure modes that would crash
when illegal debug-info was seen, to instead drop variable locations. In
practice this means that an instruction reference to a nonexistant or
illegal operand should be tolerated.

Differential Revision: https://reviews.llvm.org/D118998