[bug] Fix abs on unsigned types (taichi-dev#8476)

Issue: fixes taichi-dev#8467 

Remove abs on unsigned types

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: lin-hitonami

Diff for: taichi/transforms/simplify.cpp

@@ -133,6 +133,16 @@ class BasicBlockSimplify : public IRVisitor {
     }
   }
 
+  void visit(UnaryOpStmt *stmt) override {
+    if (stmt->op_type == UnaryOpType::abs) {
+      auto operand_type = stmt->operand->ret_type;
+      if (is_integral(operand_type) && is_unsigned(operand_type)) {
+        // abs(u) -> u
+        stmt->replace_usages_with(stmt->operand);
+        modifier.erase(stmt);
+      }
+    }
+  }
   template <typename T>
   static bool identical_vectors(const std::vector<T> &a,
                                 const std::vector<T> &b) {

Diff for: tests/python/test_abs.py

@@ -78,3 +78,13 @@ def foo(x: ti.i64) -> ti.i64:
 
     for x in [-(2**40), 0, 2**40]:
         assert foo(x) == abs(x)
+
+
+@test_utils.test()
+def test_abs_u32():
+    @ti.kernel
+    def foo(x: ti.u32) -> ti.u32:
+        return abs(x)
+
+    for x in [0, 2**20]:
+        assert foo(x) == abs(x)