tcg: Convert sub to TCGOutOpSubtract

tcg: Convert sub to TCGOutOpSubtract

Create a special subclass for sub, because two backends can
support "subtract from immediate".  Drop all backend support
for an immediate as the second operand, as we transform sub
to add during optimize.

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