[ShaderGraph] [2021.2] Fixing isNan node and re-enabling IsNaN test

[cdxntchou]

Purpose of this PR

Change the isNan node to use the isnan() HLSL function.
Re-enabling the disabled IsNan test.
The reason it was disabled was that it was waiting on a platform fix for the isnan function, that has now landed.
https://fogbugz.unity3d.com/f/cases/1300409

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Testing status

• Tested node on PC D3D11 and PC Vulkan -- returns true for nan, false for not-nan.
• Verified the isNaN test shader was producing isnan() call.
• Verified the isNaN test shader changes colors depending on the value passed in.

isNaN.mp4

Yamato:

Nightly ShaderGraph: 🟢
https://yamato.cds.internal.unity3d.com/jobs/902-Graphics/tree/sg%252Ffix%252Fnan-test2/.yamato%252Fall-shadergraph.yml%2523Nightly_ShaderGraph_2021.2/7599033/job/pipeline

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Comments to reviewers

Notes for the reviewers you have assigned.

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
Link to Yamato: https://yamato.cds.internal.unity3d.com/jobs/902-Graphics
Search for your PR branch using the sidebar on the left, then add the following segment(s) to the end of the URL (you may need multiple tabs depending on how many packages you change)

Shader Graph
/.yamato%252Fall-shadergraph.yml%2523PR_ShaderGraph_2021.2

Depending on the scope of your PR, you may need to run more jobs than what has been suggested. Please speak to your lead or a Graphics SDET (#devs-graphics-automation) if you are unsure.

[jessebarker]

LGTM