Fixed bug in verifier (#109)

* Fixed bug in verifier
Does not check variable def for unreachable codex

* Check reachability first + file test

filetests/verifier/unreachable_code.cton

@@ -0,0 +1,23 @@
+test verifier
+
+function %test() -> i32 {   ; Ok
+ebb0:
+    v0 = iconst.i32 0
+    v1 = iconst.i32 0
+    jump ebb2
+
+ebb2:
+    jump ebb4
+
+ebb4:
+    jump ebb2
+
+ebb3(v2: i32):
+    v4 = iadd.i32 v1, v2
+    jump ebb9(v4)
+
+ebb9(v7: i32):
+    v9 = iadd.i32 v2, v7
+    return v9
+
+}