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

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

Does X86 actually have a separately controllable bfloat denormal mode? This does not *control* the mode. It only informs the code generator that this is the mode. Without an underlying mode control bit, this is not correct/useful 

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