[InstCombine] foldAddWithConstant(): don't deal with non-immediate constants

All of the code that handles general constant here (other than the more
restrictive APInt-dealing code) expects that it is an immediate,
because otherwise we won't actually fold the constants, and increase
instruction count. And it isn't obvious why we'd be okay with
increasing the number of constant expressions,
those still will have to be run..

But after 2829094a
this could also cause endless combine loops.
So actually properly restrict this code to immediates.