[DebugInfo][InstrRef] Avoid crash when values optimised out late in sdag
It appears that we can emit all the instructions for a function, including
debug instructions, and then optimise some of the values out late.
Specifically, in the attached test case, an argument gets optimised out
after DBG_VALUE / DBG_INSTR_REFs are created. This confuses
MachineFunction::finalizeDebugInstrRefs, which expects to be able to find a
defining instruction, and crashes instead.
Fix this by identifying when there's no defining instruction, and
translating that instead into a DBG_VALUE $noreg.
Differential Revision: https://reviews.llvm.org/D114476