[llvm] CodeGen: Add -denormal-fp-math-bf16 flag (PR #90425)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> since all x86 bf16 instructions have only one denormal mode, which is flush to zero both for input and output.

So all the instructions are just defective? 

> If the attribute is set to other than "preserve-sign", I was considering to handle it by something like expand bf16 to f32. 

Sigh. I guess this would be the correct way to handle it 

https://github.com/llvm/llvm-project/pull/90425