Ast.GP_reg_name
type _ t = private
| W : [ `W ] t
| X : [ `X ] t
| WZR : [ `WZR ] t
| XZR : [ `XZR ] t
| WSP : [ `WSP ] t
| SP : [ `SP ] t
| LR : [ `LR ] t
| FP : [ `FP ] t