[mlir][EmitC] Fix evaluation order of expressions (#93549)

Expressions with the same precedence were not parenthesized and
therefore were possibly evaluated in the wrong order depending on the
shape of the expression tree.

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Co-authored-by: Matthias Gehre <matthias.gehre@amd.com>
Co-authored-by: Corentin Ferry <corentin.ferry@amd.com>