RFR: 8255766: Fix linux+arm64 build after 8254072

[Anton Kozlov]

On Mon, 2 Nov 2020 17:43:32 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

>> It looks OK to me, given that it compiles with gcc+msdev.
>
> Someone in hotspot team needs to determine if this is a "trivial" fix or not.

A recently introduced checked cast[1] looks to be useful here, it would allow to remove the ad-hoc assert.

[1] https://github.com/openjdk/jdk/commit/3302d3adb5fbd5729c1677d927dd4c8af1c428a4

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PR: https://git.openjdk.java.net/jdk/pull/1013