FIR2IR: wrap do-while loop in IrBlock

"so that variables declared in loop body are not visible outside of the
loop" (from commit d096f1d)

compiler/testData/ir/irText/expressions/breakContinueInLoopHeader.fir.kt.txt

@@ -64,13 +64,15 @@ fun test5() {
     i = i.inc()
     i /*~> Unit */
     var j: Int = 0
-    Inner@ do// COMPOSITE {
-    j = j.inc()
-    j
-    // }    while (when {
-      greaterOrEqual(arg0 = j, arg1 = 3) -> false
-      else -> break@Outer
-    })
+    { // BLOCK
+      Inner@ do// COMPOSITE {
+      j = j.inc()
+      j
+      // }      while (when {
+        greaterOrEqual(arg0 = j, arg1 = 3) -> false
+        else -> break@Outer
+      })
+    }
     when {
       EQEQ(arg0 = i, arg1 = 3) -> break@Outer
     }