[AMDGPU][GlobalISel] Fix legalizing image intrinsics for new types

We no longer need to increase vector size to 16 for intrinsics that use more
than 8 vgprs for addr. There is no image intrinsic that needs more than 12
so all currently existing cases will be covered. Using incorrect size was
causing an error in instruction selection because instructions were updated
to require new types (9x32, 10x32, 11x32, 12x32).

Differential Revision: https://reviews.llvm.org/D139546