RFR: 8342095: Add autovectorizer support for subword vector casts [v6]

[Jasmine Karthikeyan]

> Hi all,
> This patch adds initial support for the autovectorizer to generate conversions between subword types. Currently, when superword sees two packs that have different basic types, it discards them and bails out of vectorization. This patch changes the behavior to ask the backend if a cast between the conflicting types is supported, and keeps the pack if it is. Later, when the `VTransform` graph is built, a synthetic cast is emitted when packs requiring casts are detected. Currently, only narrowing casts are supported as I wanted to re-use existing `VectorCastX2Y` logic for the initial version, but adding more conversions is simple and can be done with a subsequent RFE. I have attached a JMH benchmark and got these results on my Zen 3 machine:
> 
> 
>                                                   Baseline                    Patch
> Benchmark                  (SIZE)  Mode  Cnt    Score    Error  Units   Score    Error  Units    Improvement
> VectorSubword.intToByte      1024  avgt   12  200.049 ± 19.787  ns/op   56.228 ± 3.535  ns/op  (3.56x)
> VectorSubword.intToShort     1024  avgt   12  179.826 ±  1.539  ns/op   43.332 ± 1.166  ns/op  (4.15x)
> VectorSubword.shortToByte    1024  avgt   12  245.580 ±  6.150  ns/op   29.757 ± 1.055  ns/op  (8.25x)
> 
> 
> I've also added some IR tests and they pass on my linux x64 machine. Thoughts and reviews would be appreciated!

Jasmine Karthikeyan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'master' into vectorize-subword
 - Address comments from review, refactor test
 - Add new conversions to benchmark
 - Fix some tests that now vectorize
 - Implement widening and address comments from review
 - Subword vectorization

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23413/files
  - new: https://git.openjdk.org/jdk/pull/23413/files/8920454d..b02408f7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23413&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23413&range=04-05

  Stats: 29959 lines in 1108 files changed: 14599 ins; 9294 del; 6066 mod
  Patch: https://git.openjdk.org/jdk/pull/23413.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23413/head:pull/23413

PR: https://git.openjdk.org/jdk/pull/23413