Adding support for Avx512Vbmi.MultiShift

[tannergooding]

This resolves #86168.

The PR ended up a bit bigger than expected because I found some issues in the GenerateHWIntrinsicTests_X86.cs file where things didn't line up or weren't getting tested as expected. The actual JIT change is just 6 lines

The tests are mostly just a refactoring, reducing the number of projects to be consistent with how other ISAs are setup. However, Avx512CD was notably not being tested and the Avx512Verify.DetectConflicts helper had a minor bug which was fixed as part of this.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[TIHan]

JIT changes are simple, looks good.

[tannergooding]

CC. @dotnet/jit-contrib for review and @dotnet/avx512-contrib as an FYI

As per the top post, the actual JIT work is small and only 6 lines (just adding entries to the tables and the perf score support). The rest of this is fixing the tests to ensure that the new APIs (and some existing APIs) are actually getting executed.