[Opt] Make merging casts int(int(x)) less aggressive (#7944)

Fixes #7915 

### Brief Summary

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### <samp>🤖 Generated by Copilot at ed6302b</samp>

Fix a bug in `alg_simp` that removed casts between signed and unsigned
integers. Add a test case in `test_optimization` to check the cast
simplification.

### Walkthrough

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### <samp>🤖 Generated by Copilot at ed6302b</samp>

* Fix a bug in algebraic simplification that incorrectly removed some
casts between signed and unsigned integers
([link](https://github.com/taichi-dev/taichi/pull/7944/files?diff=unified&w=0#diff-77d8ca8e4dc6081988bd6dddb74bb9a5485af28ce3e0b43bc06d123256695513L63-R66))
* Add a test case to verify the correctness of the cast simplification
after the bug fix
([link](https://github.com/taichi-dev/taichi/pull/7944/files?diff=unified&w=0#diff-b8b031f0789413acece482512df4af5b8419a2a2dea3624b26114bbb9b57d334R146-R155))

taichi/transforms/alg_simp.cpp

@@ -60,8 +60,10 @@ class AlgSimp : public BasicStmtVisitor {
              data_type_bits(second_cast) <= data_type_bits(first_cast);
     }
     if (is_integral(first_cast)) {
-      // int(int(a))
-      return data_type_bits(second_cast) <= data_type_bits(first_cast);
+      // int(int(a)), note it's not always equivalent when signedness differ,
+      // see #7915
+      return data_type_bits(second_cast) <= data_type_bits(first_cast) &&
+             is_signed(second_cast) == is_signed(first_cast);
     }
     // int(float(a))
     if (data_type_bits(second_cast) <= data_type_bits(first_cast) * 2) {

tests/python/test_optimization.py

@@ -143,3 +143,13 @@ def func():
     for i in range(3):
         for j in range(4):
             assert mat[i, j] == i + 1
+
+
+@test_utils.test()
+def test_casts_int_uint():
+    @ti.kernel
+    def my_cast(x: ti.f32) -> ti.u32:
+        y = ti.floor(x, ti.i32)
+        return ti.cast(y, ti.u32)
+
+    assert my_cast(-1) == 4294967295