[mlir][spirv] Respect client API requirements for 64-bit index

Vulkan requires GPU processor ID/count builtin variables to be
32-bit scalar or vector for all the cases. Similarly there
are special requirements for OpenCL. We need to make sure those
rules are respected when converting using 64bit for index.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D144819