start with the UOps.VALID spec [run_process_replay] (#6435)

* document UOps.VALID [run_process_replay]

* now the assert

tinygrad/ops.py

@@ -99,7 +99,7 @@ class UOps(HashEnum):
   CONTRACT = auto()
   SHAPETRACKER = auto()
   """
-  Defines the ShapeTracker for a buffer UOp `UOps.LOAD`, `UOps.STORE` or `UOps.CONST`.
+  Defines the ShapeTracker for a buffer UOp `UOps.LOAD`, `UOps.STORE` or `UOps.VALID`.
 
   - **`dtype`**: `None`
   - **`src`**: `Tuple[]`
@@ -169,14 +169,22 @@ class UOps(HashEnum):
 
   - **`dtype`**: The scalar DType of the value.
 
-  - **`src`**:
-    The scheduler creates a CONST with a single SHAPETRACKER UOp src: `Tuple[UOp]`.
-
-    The Lowerer replaces the SHAPETRACKER with an empty src.
-    It uses the ShapeTracker valid to create a `WHERE` UOp mask with sources: `(The actual CONST UOp, CONST 0, 0.0 or False)`
+  - **`src`**: `Tuple[]`
 
   - **`arg`**: The value.
   """
+  VALID = auto()
+  """
+  This is the first argument in a masked CONST.
+
+  - **`dtype`**: `dtypes.bool`
+  - **`src`**:
+    `Tuple[UOp]`
+      - UOps.SHAPETRACKER
+  - **`arg`**: `None`
+
+  A masked CONST is defined as `valid.where(value, 0)`.
+  """
   SPECIAL = auto()
   NOOP = auto()
   GEP = auto()
@@ -503,6 +511,7 @@ def type_verify(uops):
     if uop is UOps.LOAD and len(src) > 3 and src[3].op is UOps.ALU: assert src[3].dtype == dtypes.bool and src[2].dtype == dtype
     if uop is UOps.GEP: assert dtype == src[0].dtype.scalar(), f"GEP of {src[0].dtype=} should be {src[0].dtype.scalar()} != {dtype}"
     if uop is UOps.IF: assert dtype is None and len(src) == 2 and src[0].dtype == dtypes.bool
+    if uop is UOps.VALID: assert dtype == dtypes.bool and len(src) == 1 and src[0].op is UOps.SHAPETRACKER and arg is None
     if uop is UOps.STORE:
       assert dtype is None, f"{uop} dtype must be None, got {dtype}"
       if len(src) == 4: assert src[3].dtype == dtypes.bool or src[3].op is UOps.IF, f"bad gate {src[3]}"