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