[llvm] SelectionDAG: Use qNaN constant if FCANONICALIZE not LegalOrCustom (PR #104564)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm How does AMDGPU backend prevent mul * 1.0 from being optimized away?

It doesn't. This is why the canonicalize intrinsic exists.

https://github.com/llvm/llvm-project/pull/104564