remove attempt to optimize using 'if' statement as the last one in a loop which created far more problems that it fixed

#KT-1978 Fixed

compiler/testData/codegen/regressions/kt1978.kt

@@ -0,0 +1,10 @@
+fun aa(vararg a : String): String = a[0]
+
+fun box(): String {
+    var result: String = ""
+    var i = 1
+    while (3 > i++) {
+        result = aa(if (true) "OK" else "fail")
+    }
+    return result
+}