vim-patch:8.2.0924: cannot save and restore a register properly

Problem:    Cannot save and restore a register properly.
Solution:   Add getreginfo() and make setreg() accept a dictionary. (Andy
            Massimino, closes vim/vim#3370)
bb861e293e

Cherry-pick eval.txt changes for getreginfo() from:
6aa57295cf
207f009326

src/nvim/ops.c

@@ -804,12 +804,6 @@ bool valid_yank_reg(int regname, bool writing)
   return false;
 }
 
-typedef enum {
-  YREG_PASTE,
-  YREG_YANK,
-  YREG_PUT,
-} yreg_mode_t;
-
 /// Return yankreg_T to use, according to the value of `regname`.
 /// Cannot handle the '_' (black hole) register.
 /// Must only be called with a valid register name!
@@ -3650,6 +3644,12 @@ int get_register_name(int num)
   }
 }
 
+/// @return the index of the register "" points to.
+int get_unname_register(void)
+{
+  return y_previous == NULL ? -1 : (int)(y_previous - &y_regs[0]);
+}
+
 /*
  * ":dis" and ":registers": Display the contents of the yank registers.
  */