opt: Start optimizing dead code a bit

If a condition is always zero/non-zero we can omit the
then or else code.  This is complicated a bit by having to
deal with labels that might make such code reachable without
us yet knowing during parsing.

tests/tests2/87_dead_code.expect

@@ -0,0 +1,18 @@
+timeout=2
+timeout=1
+boo
+yeah
+twice
+once
+twice
+SEtwice
+SEonce
+SEtwice
+twice2
+once2
+twice2
+twice3
+once3
+twice3
+caseok
+caseok2