Handle conditional expressions with non-identical types

[zhiyuanshi]

Source:

if True
   then { status = "placed" }
   else { status = "cancelled", cancelledAt = "2015-03-10" }

Output:

should_compile[hotfix/issue-98] % f2j TypingIf.sf -r
TypingIf using [Naive]
Compiling to Java source code ( ./TypingIf.java )
./TypingIf.java:13: error: incompatible types: Tuple2 cannot be converted to String
      ifres1 = x2;
               ^
1 error

System FI:

if True 
   then { status = "placed" } 
   else ({ status = "cancelled" } ,, { cancelledAt = "2015-03-10" })

FCore:

if True 
   then "placed" 
   else ("cancelled", "2015-03-10")

Simplify did not generate valid a System F expression in this case.