Add a simple_gvn test that includes some basic control flow.

filetests/simple_gvn/basic.cton

@@ -19,3 +19,23 @@ ebb0(v0: i32, v1: i32):
 ; check: v6 = iadd $v4, $v4
     return v6
 }
+
+function %redundancies_on_some_paths(i32, i32, i32) -> i32 {
+ebb0(v0: i32, v1: i32, v2: i32):
+    v3 = iadd v0, v1
+    brz v3, ebb1
+    v4 = iadd v0, v1
+    jump ebb2(v4)
+; check: jump ebb2(v3)
+
+ebb1:
+    v5 = iadd v0, v1
+    jump ebb2(v5)
+; check: jump ebb2(v3)
+
+ebb2(v6: i32):
+    v7 = iadd v0, v1
+    v8 = iadd v6, v7
+; check: v8 = iadd v6, v3
+    return v8
+}