doc: add more sample ir and graph files

doc/sample/ir/for-loop-3.mie

@@ -0,0 +1,20 @@
+~func.func() ({
+^entry(%buffer: memref<1024*f32>, %lb: index, %ub: index, %step: index):
+	; Initial sum set to 0.
+	%sum.0 = ~f32.constant() {value = 0.0 : f32} : () -> f32
+
+	; iter_args binds initial values to the loop's region arguments.
+	%sum = ~scf.for(%lb, %ub, %step) -> (f32) {
+	^for.entry(%iv: index, %sum.iter: f32):
+		%t = ~memref.load(%buffer, %iv) : (memref<1024*f32>, index) -> f32
+		%sum.next = Zarith.addf(%sum.iter, %t) : (f32, f32) -> f32
+		; Yield current iteration sum to next iteration %sum.iter or to %sum
+		; if final iteration.
+		%iv.next = ~arith.addi(%iv, %step) : (index, index) -> index
+		%stop = ~arith.cmpi(%iv.next, %ub) {predicate = 9 : i64} : (index, index) -> i1
+		~cf.br-cond(%stop) [ ^for.end, ^for.entry:(%iv.next: index, %sum.next: f32) ] : () -> ()
+	}
+^for.end:
+	~func.return(%sum) : (f32) -> ()
+}) {sym_name = "reduce"} : (memref<1024*i32>, index, index, index) -> f32
+