[flang][CodeGen] Run PreCGRewrite on omp reduction declare ops (#84954)

OpenMP reduction declare operations can contain FIR code which needs to
be lowered to LLVM. With array reductions, these regions can contain
more complicated operations which need PreCGRewriting. A similar extra
case was already needed for fir::GlobalOp.

OpenMP array reductions 3/6
Previous PR: https://github.com/llvm/llvm-project/pull/84953
Next PR: https://github.com/llvm/llvm-project/pull/84955