gallivm: honor d3d10 floating point rules for shadow comparisons
gallivm: honor d3d10 floating point rules for shadow comparisons

d3d10 specifies ordered comparisons for everything but not_equal which is
unordered (http://msdn.microsoft.com/en-us/library/windows/desktop/cc308050.aspx).
OpenGL probably doesn't care.

Reviewed-by: Zack Rusin <6ab0df0692bc027e828ec9e4e9fbe8fa1a7842a8@vmware.com>

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