tcg: Add generic vector ops for constant shifts

tcg: Add generic vector ops for constant shifts

Opcodes are added for scalar and vector shifts, but considering the
varied semantics of these do not expose them to the front ends.  Do
go ahead and provide them in case they are needed for backend expansion.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>