Change iftable to never be null

Simplifies code generation by removing a null check. The null case
is rare.

Ritzperf code size: 13107624 -> 13095336

Also addressed comments from previous CL.

Bug: 32577579

Test: test-art-host, run ritzperf both with CC

Change-Id: I2b31e800867112869d7f0643e16c08826296979e
diff --git a/test/586-checker-null-array-get/src/Main.java b/test/586-checker-null-array-get/src/Main.java
index e0782bc..0ea7d34 100644
--- a/test/586-checker-null-array-get/src/Main.java
+++ b/test/586-checker-null-array-get/src/Main.java
@@ -100,7 +100,7 @@
   /// CHECK-DAG:                     Return [<<ArrayGet2>>]
   public static float test1() {
     Test1 test1 = getNullTest1();
-    Test2 test2 = getNullTest2();;
+    Test2 test2 = getNullTest2();
     int[] iarr = test1.iarr;
     float[] farr = test2.farr;
     iarr[0] = iarr[1];