isl_pw_*_scale{,_down}_val: always negate type on negative scaling factor
isl_pw_*_scale{,_down}_val: always negate type on negative scaling factor

It is not clear if the type ever makes a difference
on an empty (i.e., zero) reduction, but there is no reason
to make an exception for this case.
Always negating the type is also consistent with how this
is handled by isl_pw_*_mul_isl_int.

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl