Regalloc: fresh variables may have wrong kind

[vbgl]

export
fn add_stack(reg u64 x) {
  stack u64 s;
  s = 0;
  ?{}, _ = #ADD(s, x);
}

The Regalloc.fill_in_missing_names function creates a variable of kind reg for the _ destination, which later leads to asmgen failure:

"bug_679.jazz", line 5 (2-22): internal compilation error in function add_stack: asmgen: (compile_arg) not compatible asm_arg