don't prune first block and emit pass for empty blocks

As title

numba_rvsdg/core/datastructures/ast_transforms.py

@@ -190,7 +190,7 @@ def prune_empty(self) -> set[WritableASTBlock]:
         """Prune empty blocks from the CFG."""
         empty = set()
         for name, block in list(self.items()):
-            if not block.instructions:
+            if not block.instructions and name != "0":
                 empty.add(self.pop(name))
                 # Empty blocks can only have a single jump target.
                 it = block.jump_targets[0]
@@ -724,6 +724,9 @@ def codegen(self, block: Any) -> MutableSequence[ast.AST]:
                 )
                 if_node = ast.If(test, body, orelse)
                 return block.tree[:-1] + [if_node]
+            elif block.fallthrough and len(block.tree) == 0:
+                # Am empty block, do nothing.
+                return [ast.Pass()]
             elif block.fallthrough and type(block.tree[-1]) is ast.Return:
                 # The value of the ast.Return could be either None or an
                 # ast.AST type. In the case of None, this refers to a plain

numba_rvsdg/tests/test_ast_transforms.py

@@ -621,39 +621,28 @@ def function() -> int:
     def test_if_in_while_without_else(self):
         def function(i) -> int:
             while i < 10:
-                if i > 2:
-                    s = 123
                 i += 1
-            return s
+            return i
 
         expected = {
+            "0": {"instructions": [], "jump_targets": ["1"], "name": "0"},
             "1": {
                 "instructions": ["i < 10"],
                 "jump_targets": ["2", "3"],
                 "name": "1",
             },
             "2": {
-                "instructions": ["i > 2"],
-                "jump_targets": ["5", "7"],
+                "instructions": ["i += 1"],
+                "jump_targets": ["1"],
                 "name": "2",
             },
             "3": {
-                "instructions": ["return s"],
+                "instructions": ["return i"],
                 "jump_targets": [],
                 "name": "3",
             },
-            "5": {
-                "instructions": ["s = 123"],
-                "jump_targets": ["7"],
-                "name": "5",
-            },
-            "7": {
-                "instructions": ["i += 1"],
-                "jump_targets": ["1"],
-                "name": "7",
-            },
         }
-        self.compare(function, expected, empty={"0", "4", "6"})
+        self.compare(function, expected, empty={"4"}, arguments=[(0,)])
 
     def test_while_in_if(self):
         def function(y: int) -> int: