Fixup global registers so that they assign to locations within the RegTable if they aren't pinned for the current target.

We map STG registers onto appropriate CmmExprs. Either they map to real machine registers or stored as offsets from BaseReg. Given a GlobalReg, get_GlobalReg_addr always produces the register table address for it.

The Stream instantiation used for code generation. Note the underlying monad is UniqDSMT IO, where UniqDSMT is a transformer that propagates a deterministic unique supply (essentially an incrementing counter) from which new uniques are deterministically created during the code generation stages following StgToCmm. See Note [Object determinism].