Add constant folding of ifs.

compiler/front-end.rkt

@@ -204,6 +204,13 @@
                  (substitute-child! p node res)))]
              [else
               (fail!)]))]
+    [(if* _ cs) ; if result of test is known, keep only the used branch
+     (for-each constant-fold! cs) ; fold each branch
+     (match node
+       [(if* p `(,(cst _ '() val) ,thn ,els))
+        (substitute-child! p node (if val thn els))]
+       [_
+        (void)])] ; nothing to do, we folded children already
     [_
      (for-each constant-fold! (node-children node))]))
 

tests/succeed/constant-folding.expected

@@ -1 +1,4 @@
 14
+1
+1
+2

tests/succeed/constant-folding.scm

@@ -3,3 +3,7 @@
 ;; Also, the current test harness cannot detect if constant folding does
 ;; not happen. This test will only fail if there's an error somewhere.
 (displayln (#%+ 2 (#%mul-non-neg 3 4)))
+
+(displayln (if #t 1 2)) ; all these should be a single constant, no if
+(displayln (if 3  1 2))
+(displayln (if #f 1 2))