Coerce loop body to Unit.

compiler/testData/ir/irText/regressions/coercionInLoop.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val a = DoubleArray(5)
+    val x = a.iterator()
+    var i = 0
+    while (x.hasNext()) {
+        if (a[i] != x.next()) return "Fail $i"
+        i++
+    }
+    return "OK"
+}