Update BitArray.CopyTo to use cross-platform intrinsics

[Copilot]

Fixes #114818 (comment)

This PR updates BitArray.CopyTo to use cross-platform intrinsic APIs instead of platform-specific ones, making the code more portable across different hardware architectures.

Changes

AVX512 path (Vector512)

• Avx512BW.IsSupported → Vector512.IsHardwareAccelerated
• Avx512BW.Shuffle(x, y) → Vector512.Shuffle(x, y)
• Avx512F.And(x, y) → x & y
• Avx512BW.Min(x, y) → Vector512.Min(x, y)
• Avx512F.Store(x, y) → y.Store(x)

AVX2 path (Vector256)

• Avx2.IsSupported → Vector256.IsHardwareAccelerated
• Avx2.Shuffle(x, y) → Vector256.Shuffle(x, y)
• Avx2.And(x, y) → x & y
• Avx2.Min(x, y) → Vector256.Min(x, y)
• Avx.Store(x, y) → y.Store(x)

Unified Vector128 path

• Consolidated SSSE3, PackedSimd, and AdvSimd paths into a single unified Vector128 implementation
• Ssse3.IsSupported → Vector128.IsHardwareAccelerated
• Ssse3.Shuffle(x, y) → Vector128.Shuffle(x, y) (internally uses PackedSimd.Swizzle on WASM and AdvSimd.Arm64.VectorTableLookup on ARM)
• Sse2.And(x, y) / PackedSimd.And(x, y) / AdvSimd.And(x, y) → x & y
• Sse2.Min(x, y) / PackedSimd.Min(x, y) / AdvSimd.Min(x, y) → Vector128.Min(x, y)
• Sse2.Store(x, y) / PackedSimd.Store(x, y) / AdvSimd.Arm64.StorePair(x, y, z) → y.Store(x)
• Removed ~63 lines of redundant platform-specific code

Benefits

• Cross-platform compatibility: These cross-platform intrinsics provide the same functionality as the platform-specific ones but allow the JIT to optimize for the actual hardware at runtime
• Improved maintainability: Single Vector128 implementation instead of three separate paths (SSSE3, PackedSimd, AdvSimd)
• Better ARM performance: Using VectorTableLookup instead of manually chained ZipLow/ZipHigh instructions
• Reduced code duplication: Consolidated multiple platform-specific paths into unified implementations

Testing

All existing System.Collections tests pass (33,438 tests, 0 failures).

Fixes #116079

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update BitArray.CopyTo to use the xplat intrinsics</issue_title>
<issue_description>As per #114818 (comment), BitArray.CopyTo could benefit from using the cross platform intrinsic APIs rather than the platform specific ones.

This can be achieved by swapping out these API calls:

• Avx512BW.IsSupported -> Vector512.IsHardwareAccelerated
• Avx512BW.Shuffle(x, y) -> Vector512.Shuffle(x, y)
• Avx512F.And(x, y) -> x & y
• Avx512BW.Min(x, y) -> Vector512.Min(x, y)
• Avx512F.Store(x, y) -> y.Store(x)

Similar changes could also be made to the Avx2 path (using Vector256) and so on.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #116079

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Pull Request Overview

This PR updates BitArray.CopyTo to use cross-platform intrinsic APIs instead of platform-specific ones, improving code portability across different hardware architectures. The changes replace platform-specific AVX512 and AVX2 intrinsics with their cross-platform equivalents.

Key changes:

• Replace AVX512BW/AVX512F intrinsics with Vector512 cross-platform APIs
• Replace AVX2/AVX intrinsics with Vector256 cross-platform APIs
• Use hardware acceleration detection and operator overloads for better portability