[FIR2IR] Don't generate excess IR block from FirSingleExpressionBlock

This is needed to unify K1 and K2 behavior.

#KT-65064
#KT-63781 Fixed

compiler/testData/ir/irText/expressions/ifWithLoop.kt

@@ -0,0 +1,42 @@
+fun topLevelMethod() {
+    var z = 1
+
+    if(true) { for (i in 0..2) z += i } else { for (i in 0..3) z += i }
+    if(true) for (i in 0..4) z += i else for (i in 0..5) z += i
+
+    if(true) { while(false) z += 6 } else { while(false) z += 7 }
+    if(true) while(false) z += 8 else while(false) z += 9
+
+    if(true) { do { z += 10 } while(false) } else { do { z += 11 } while(false) }
+    if(true) do { z += 12 } while(false) else do { z += 13 } while(false)
+
+    when(z) {
+        1 -> { for (i in 0..14) z += i }
+        else -> { for (i in 0..15) z += i }
+    }
+
+    when(z) {
+        1 -> for (i in 0..16) z += i
+        else -> for (i in 0..17) z += i
+    }
+
+    when(z) {
+        1 -> { while(false) z += 18 }
+        else -> { while(false) z += 19 }
+    }
+
+    when(z) {
+        1 -> while(false) z += 20
+        else -> while(false) z += 21
+    }
+
+    when(z) {
+        1 -> { do { z += 22 } while(false) }
+        else -> { do { z += 23 } while(false) }
+    }
+
+    when(z) {
+        1 -> do { z += 24 } while(false)
+        else -> do { z += 25 } while(false)
+    }
+}