Use dup to convert a non-constant scalar to a third vector.

Add addition, multiplication, and logical operations with an immediate.
Add addition, subtraction, multiplication, and logical operations with
a non-constant scalar.  Allow for the front-end to build operations in
which the scalar operand comes first.

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