[TVMScript] Use new variable frame in If/Then/Else (#14250)

Previously, while TVMScript introduces a new scope for other
contexts (e.g. `for`, `while`, `with`, etc), the `if` and `else`
blocks did not introduce a new scope.  This caused erroneous parsing
errors if the `if` and `else` blocks each contained a variable with
the same name.  Added a `self.var_table.with_frame()` context resolves
this issue.

python/tvm/script/parser/tir/parser.py

@@ -433,10 +433,12 @@ def visit_if(self: Parser, node: doc.If) -> None:
     with self.var_table.with_frame():
         with T.If(self.eval_expr(node.test)):
             with T.Then():
-                self.visit_body(node.body)
+                with self.var_table.with_frame():
+                    self.visit_body(node.body)
             if node.orelse:
                 with T.Else():
-                    self.visit_body(node.orelse)
+                    with self.var_table.with_frame():
+                        self.visit_body(node.orelse)
 
 
 @dispatch.register(token="tir", type_name="Assert")

tests/python/unittest/test_tvmscript_roundtrip.py

@@ -3623,6 +3623,19 @@ def main(A: T.handle, B: T.handle):
     return main
 
 
+def if_then_else_var():
+    @T.prim_func
+    def main(n: T.int32):
+        if n == 0:
+            x = 5
+            T.evaluate(x)
+        else:
+            x = 10
+            T.evaluate(x)
+
+    return main
+
+
 def tvm_shfl_builtins():
     @T.prim_func
     def func(
@@ -3740,6 +3753,7 @@ def func(
     let_stmt_value,
     string_stride,
     merge_shape_var_def,
+    if_then_else_var,
     tvm_shfl_builtins,
 )