[spirv] Fix bitwidth emulation for Workgroup storage class

If Int16 is not available, 16-bit integers inside Workgroup storage
class should be emulated via 32-bit integers. This was previously
broken because the capability querying logic was incorrectly
intercepting all storage classes where it meant to only handle
interface storage classes. Adjusted where we return to fix this.

Differential Revision: https://reviews.llvm.org/D85308