Typechecker logic fixed for one-branch ifs and 'return x' for Unit-returning functions

idea/testData/cfg/Nonlocals.instructions

@@ -36,7 +36,7 @@ l5:
     <END>
 =====================
 == nonlocals1 ==
-fun nonlocals1(a : Boolean, b : Boolean) : Unit {
+fun nonlocals1(a : Boolean, b : Boolean) : Any? {
     if (a)
         return 1
     1

idea/testData/cfg/Nonlocals.jet

@@ -1,4 +1,4 @@
-fun nonlocals1(a : Boolean, b : Boolean) : Unit {
+fun nonlocals1(a : Boolean, b : Boolean) : Any? {
     if (a)
         return 1
     1

idea/testData/checker/FunctionReturnTypes.jet

@@ -3,6 +3,8 @@
 fun unitEmptyInfer() {}
 fun unitEmpty() : Unit {}
 fun unitEmptyReturn() : Unit {return}
+fun unitIntReturn() : Unit {return <error>1</error>}
+fun unitUnitReturn() : Unit {return ()}
 fun unitShort() : Unit = ()
 fun unitShortConv() : Unit = 1
 fun unitShortNull() : Unit = null