RFR: 8257165: C2: Improve box elimination for vector masks and shuffles

[Vladimir Kozlov]

On Thu, 26 Nov 2020 13:17:05 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Introduce VectorMask/VectorShuffle-specific transformations to reduce reboxing by eliminating `VectorBox`/`VectorUnbox` pairs. 
> 
> It's a trivial transformation when the types on both ends perfectly match, but when type mismatch occurs there are additional steps needed (see `PhaseVector::expand_vunbox_node()` for more details on vector unboxing) .  
> 
> Testing:
> - `jdk/incubator/vector` tests w/ different flag combinations (no flags, `-Xcomp`, `-XX:+DeoptimizeALot`); 
> - tier1-4

Okay.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1457