assert Kernel ast.op to be Ops.SINK [pr] (tinygrad#9539)

rest of the code assumes self.ast is defined anyway

Author: chenyuxyz

tinygrad/codegen/kernel.py

@@ -37,7 +37,8 @@ def fix_axes(self, removed_axis:int): # adjust the TC axes if necessary when a d
 
 class Kernel:
   def __init__(self, ast:UOp, opts:Optional[Renderer]=None):
-    if ast.op is Ops.SINK: self.ast = ast
+    assert ast.op is Ops.SINK, ast.op
+    self.ast = ast
 
     self.opts = opts if opts is not None else Device[Device.DEFAULT].renderer
     # verify AST matches the spec