Update model solution.

- Stop calculating operands for & operators.
- Evaluate the lhs of compound operators only once.
- Add `side-effect.c` example.

examples/ir4/side_effect.ir

@@ -0,0 +1,29 @@
+var i32 @g = 0
+
+fun i32* @foo () {
+init:
+  bid: b0
+  allocations: 
+
+
+block b0:
+  %b0:i0:i32 = load @g:i32*
+  %b0:i1:i32 = add %b0:i0:i32 10:i32
+  %b0:i2:unit = store %b0:i1:i32 @g:i32*
+  ret @g:i32*
+}
+
+fun i32 @main () {
+init:
+  bid: b0
+  allocations: 
+
+
+block b0:
+  %b0:i0:i32* = call @foo:[ret:i32* params:()]*()
+  %b0:i1:i32 = load %b0:i0:i32*
+  %b0:i2:i32 = add %b0:i1:i32 1:i32
+  %b0:i3:unit = store %b0:i2:i32 %b0:i0:i32*
+  %b0:i4:i32 = load @g:i32*
+  ret %b0:i4:i32
+}