[BACKEND][FRONTEND] Fix problems with test_matmul (#973)

1. Handle induction variable when step is negative
2. Restore async_wait that accidentally deleted
3. Add missing induction variable in prefetch
4. Add device property functions

Co-authored-by: Philippe Tillet <Phil.Tillet@gmail.com>

python/tests/test_vecadd.py

@@ -68,7 +68,7 @@ def test_vecadd_scf_mask(shape, num_warps, block_size, iter_size):
         @num_elements: number of elements
         '''
         pid = tl.program_id(axis=0)
-        for i in range(math.ceil(block_size / iter_size)):
+        for i in range(tl.cdiv(block_size, iter_size)):
             # TODO: a bug here, if put the offset outside the forloop, there will be a GPU mis-aligned error.
             offset = pid * block_size + tl.arange(0, iter_size)
             x_ptrs = x_ptr + offset