FIR DFA: make continue jump to condition entry, not loop entry

It's also not a backwards jump in do-while, unless it's in the loop's
condition, which is a stupid "feature" IMO. As you can probably tell
from the comments added in this commit.

compiler/testData/diagnostics/tests/smartCasts/loops/doWhileEarlyContinue.fir.kt

@@ -1,13 +0,0 @@
-fun x(): Boolean { return true }
-
-fun y(): Boolean { return false }
-
-public fun foo(p: String?): Int {
-    do {
-        if (y()) continue
-        // We do not always reach this statement
-        p!!.length
-    } while (!x())
-    // Here we have do while loop but p is still nullable due to continue before
-    return p.length
-}

compiler/testData/diagnostics/tests/smartCasts/loops/doWhileEarlyContinue.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 fun x(): Boolean { return true }
 
 fun y(): Boolean { return false }

compiler/testData/diagnostics/tests/smartCasts/loops/nestedDoWhileWithLongContinue.fir.kt

@@ -5,10 +5,10 @@ public fun foo(p: String?, r: String?): Int {
         do {
             p!!.length
             if (!x()) continue@outer
-        } while (r == null)  
+        } while (r == null)
     } while (!x())
     // Auto cast NOT possible due to long continue
-    r.length
+    r<!UNSAFE_CALL!>.<!>length
     // Auto cast possible
     return p.length
 }

compiler/testData/diagnostics/tests/smartCasts/loops/nestedDoWhileWithLongContinue.kt

@@ -5,7 +5,7 @@ public fun foo(p: String?, r: String?): Int {
         do {
             p!!.length
             if (!x()) continue@outer
-        } while (r == null)  
+        } while (r == null)
     } while (!x())
     // Auto cast NOT possible due to long continue
     r<!UNSAFE_CALL!>.<!>length