Add a flag for the upstream global reassociation algorithm change

[lizhengxing]

This PR (https://github.com/microsoft/DirectXShaderCompiler/pull/6598) pulls the upstream global reassociation algorithm change in DXC and can reduce redundant calculations obviously.

However, from the testing result of a large offline suite of shaders, some shaders got worse compilation results and couldn't benefit from this upstream change.

This PR adds a flag for the upstream global reassociation change. It would be easier to roll back if a shader get worse compilation result due to this upstream change.

This is part 2 of the fix for #6593.

[github-actions[bot]]

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