Update rustc_codegen_gcc rotate operation document

[notJoon]

Description

This PR resolves a TODO comment in the rustc_codegen_gcc backend by documenting that the rotate operations (rotate_left and rotate_right) already implement the optimized branchless algorithm from comment.

The existing implementation already uses the optimal branchless rotation pattern:

• For left rotation: (x << n) | (x >> (-n & (width-1)))
• For right rotation: (x >> n) | (x << (-n & (width-1)))

This pattern avoids branches and generates efficient machine code across different platforms, which was the goal mentioned in the original TODO.

Changes

• Removed the TODO comment that suggested implementing the algorithm from https://blog.regehr.org/archives/1063

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Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

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[lcnr]

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