[llvm] [DAG][AMDGPU] Add lit tests for FMA combining with freeze and nnan variants (PR #142628)

[Jay Foad via llvm-commits]

jayfoad wrote:

> After this PR #142345, combining `freeze` on `fmul` (without `nnan`) followed by `fadd` or `fsub` into a single `fma` is supported. This patch adds lit tests to verify the optimization behavior for both nnan and non-nnan variants.

Thanks - but making this work in the presence of ninf and nnan is pretty important too.

https://github.com/llvm/llvm-project/pull/142628