[llvm] [ConstantFolding] Add flag to disable call folding (PR #140270)

[Lewis Crawford via llvm-commits]

LewisCrawford wrote:

The @llvm.fabs intrinsic is lowered correctly in NVPTX. It's the target-specific @llvm.nvvm.fabs intrinsic that is allowed have weird NaN behaviour.

I've merged it for now to unblock adding more NVVM-intrinsic constant-folding in https://github.com/llvm/llvm-project/pull/141233 , but I'll take a look at whether AllowNonDeterminstic might work here in a follow-up patch.

https://github.com/llvm/llvm-project/pull/140270