[GlobalISel] using low-level type s0 as an opaque token type

In RFC D158147, we lower the LLVM token type to the low-level type s0. This is used to represent convergence control tokens in GMIR. The convergence control tokens are useful only in SSA form prior to instruction selection. Inviting feedback on this interpretation of a scalar register type with no size.

Convergence control tokens in LLVM IR are described here: