nir/spirv: Insert movs around image intrinsics

nir/spirv: Insert movs around image intrinsics

Image intrinsics always take a vec4 coordinate and always return a vec4.
This simplifies the intrinsics a but but also means that they don't
actually match the incomming SPIR-V.  In order to compensate for this, we
add swizzling movs for both source and destination to get the right number
of components.