target-microblaze: Simplify address computation using tcg_gen_addi_i32()

Simplify address computation using tcg_gen_addi_i32().
tcg_gen_addi_i32() already optimizes the case when the
immediate is zero.

No functional change.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>