[llvm] [mlir] [APFloat] Don't turn a Float8E8M0FNU NaN into an Inf on conversion (PR #214919)

[曾鈜寬 Tseng Hung Kuan via llvm-commits]

Tim096 wrote:

Confirming that I have read the LLVM Developer Policy, the Code Review
Policy, and the AI Tool Use Policy.

I am the author of this change and I am accountable for it. I have read
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during review.

https://github.com/llvm/llvm-project/pull/214919