RFR: AESCrypt.implEncryptBlock/AESCrypt.implDecryptBlock intrinsics assume non null inputs

[Roland Westrelin]

> The use of must_be_not_null on non-Shenandoah paths concerns me a little, but it seems to be in line
> with what we are already doing:
>
> https://builds.shipilev.net/patch-openjdk-shenandoah-jdk10/2017-10-20-v174-vs-74700c8e39e9/src/hotspot/share/opto/library_call.cpp.sdiff.html
>
> This, BTW, makes me think if we want to put must_be_not_null inside shenandoah_*_barrier to make it
> perfectly neutral.

At some point, I intend to propose the must_be_not_null() I added to the
intrinsics upstream, independently of Shenandoah.

Roland.