tcg/optimize: With two const operands, prefer 0 in arg1

tcg/optimize: With two const operands, prefer 0 in arg1

For most binary operands, two const operands fold.
However, the add/sub carry opcodes have a third input.
Prefer "reg, zero, const" since many risc hosts have a
zero register that can fit a "reg, reg, const" insn format.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>