rename register for IsNull opcode

Now it has the same name as NotNull, so it is easier to write macros

core/translate/expr.rs

@@ -448,7 +448,7 @@ pub fn translate_condition_expr(
             let cur_reg = program.alloc_register();
             translate_expr(program, Some(referenced_tables), expr, cur_reg, resolver)?;
             program.emit_insn(Insn::IsNull {
-                src: cur_reg,
+                reg: cur_reg,
                 target_pc: condition_metadata.jump_target_when_false,
             });
         }

core/vdbe/builder.rs

@@ -310,7 +310,7 @@ impl ProgramBuilder {
                 Insn::IdxGT { target_pc, .. } => {
                     resolve(target_pc, "IdxGT");
                 }
-                Insn::IsNull { src: _, target_pc } => {
+                Insn::IsNull { reg: _, target_pc } => {
                     resolve(target_pc, "IsNull");
                 }
                 _ => continue,

core/vdbe/explain.rs

@@ -1034,14 +1034,14 @@ pub fn insn_to_str(
                 0,
                 "".to_string(),
             ),
-            Insn::IsNull { src, target_pc } => (
+            Insn::IsNull { reg, target_pc } => (
                 "IsNull",
-                *src as i32,
+                *reg as i32,
                 target_pc.to_debug_int(),
                 0,
                 OwnedValue::build_text(Rc::new("".to_string())),
                 0,
-                format!("if (r[{}]==NULL) goto {}", src, target_pc.to_debug_int()),
+                format!("if (r[{}]==NULL) goto {}", reg, target_pc.to_debug_int()),
             ),
             Insn::ParseSchema { db, where_clause } => (
                 "ParseSchema",

core/vdbe/insn.rs

@@ -510,7 +510,7 @@ pub enum Insn {
     /// Check if the register is null.
     IsNull {
         /// Source register (P1).
-        src: usize,
+        reg: usize,
 
         /// Jump to this PC if the register is null (P2).
         target_pc: BranchOffset,

core/vdbe/mod.rs

@@ -2389,8 +2389,8 @@ impl Program {
                     cursors.get_mut(*cursor_id).unwrap().take();
                     state.pc += 1;
                 }
-                Insn::IsNull { src, target_pc } => {
-                    if matches!(state.registers[*src], OwnedValue::Null) {
+                Insn::IsNull { reg, target_pc } => {
+                    if matches!(state.registers[*reg], OwnedValue::Null) {
                         state.pc = target_pc.to_offset_int();
                     } else {
                         state.pc += 1;