[llvm] [LLParser] Support identifiers like `qnan` and `pinf` for special FP values (PR #102790)

[Nikita Popov via llvm-commits]

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nikic wrote:

Can you please also add a normal lit test in llvm/test/Assembler? Just an llvm-as | llvm-dis round-trip.

https://github.com/llvm/llvm-project/pull/102790