tcg: Do not require both rotr and rotl from the backend

tcg: Do not require both rotr and rotl from the backend

Many host architectures do not implement both rotate right
and rotate left and require the compiler to negate the
shift count to rotate the opposite direction.  We have been
requiring the backend to perform this transformation.
Do this during opcode expansion so that the next patch
can drop support where possible in the backend.

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