Compensate in compiler for verifier shortcomings.

The verifier does not differentiate zero and null, so a move-object
of zero can be used as a non-object later on.

Change the compiler to ignore the object conversion when the input
is zero or a phi (which might just hold zeros). The type propagation
will then do proper inferencing of the types.

Also remove some stalled comments in ssa_builder.cc.

bug:31313170
test: dex2oat b31313170.apk
test: run-test 800
test: m test-art-host-run-test
Change-Id: I579d667415a7decf8ff2c2238dae4c13eec5d0e0