simple drop image valid case (#6548)

* simple drop image valid case

started unit test, 530 -> 473 valids

* cleanup

test/unit/test_image_valid.py

@@ -0,0 +1,34 @@
+import unittest
+
+from tinygrad.codegen.uopgraph import linearize_uop, full_graph_rewrite
+from tinygrad.dtype import dtypes
+from tinygrad.ops import UOp, UOps, BinaryOps
+
+def render(image_shape, valid:UOp, idx:UOp) -> str:
+  uops = linearize_uop(full_graph_rewrite(UOp(UOps.LOAD, dtypes.float.vec(4), (
+    UOp(UOps.DEFINE_GLOBAL, dtypes.imagef(image_shape), arg=0),
+    idx,
+    UOp(UOps.VECTORIZE, dtypes.float.vec(4), src=(UOp.const(dtypes.float, 0),)*4),
+    valid
+  )).sink()))
+  from tinygrad.renderer.cstyle import OpenCLRenderer
+  class TestRenderer(OpenCLRenderer):
+    code_for_op = {**OpenCLRenderer().code_for_op, BinaryOps.IDIV: lambda a,b,dtype: f"({a}//{b})"}
+  fxn = TestRenderer().render("", uops)
+  return fxn.split("float4 val0 = ")[1].split(";")[0]
+
+def Variable(expr, nmax):
+  return UOp(UOps.SPECIAL, dtypes.int, (), (expr, nmax))
+
+class TestValidSimplification(unittest.TestCase):
+  def test_idx_lt_0(self):
+    # (idx1 * (-1) < 0) ? (..., idx1-1) : 0 can drop the valid
+    gidx0 = Variable("gidx0", 32)
+    gidx1 = Variable("gidx1", 32)
+    self.assertEqual(
+      render((10, 10, 4), (gidx1*(-1)).lt(0), UOp(UOps.VECTORIZE, dtypes.int.vec(2), (gidx0, gidx1-1))),
+      "read_imagef(data0, smp, (int2)(gidx0,(gidx1+(-1))))"
+    )
+
+if __name__ == '__main__':
+  unittest.main()