RFR: 8262355: Support for AVX-512 opmask register allocation. [v8]

[Jatin Bhateja]

On Tue, 16 Mar 2021 21:19:21 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> I think post-loop vectorization handling currently expects mask to be present in K1, I just pulled it out till the instruction level,  RA allocates opmask registers in reverse order i.e. K7-K1, thus temps should all be allocated K7 registers first.
>
> It is just a coincidence. Considering there's a way to produce an arbitrary number of predicates using `VectorMask`, I'd prefer to see a more robust solution that won't cause us problems in the future.
> Please, exclude `k1` from `kReg` then.

For the time being I have removed K1 from allocable set, till we fix post-loop multi-versioning.

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PR: https://git.openjdk.java.net/jdk/pull/2768