[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

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