https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/aarch64.ad#L3398 says

  /// mov envcodings

I think "envcodings" is a typo for "encodings".

I also think the triple-slash end-of-line comment is a typo: a double-slash would do, and I don't see any other uses of triple-slash comments to indicate that it might be trying to tell me something.

It looks like that code has been unchanged since the aarch64 C1 and C2 compilers were added 9 years ago.