Use AVX2 for bitmap intersection stores

[perfloop-agent]

Description

Use AVX2 to materialize bitmap-container AND results on amd64 CPUs that do not use the AVX-512 bitmap-operation path.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement
• Code refactoring
• Documentation update
• Test improvements
• Build/CI changes

Changes Made

What was changed?

• Added four-word AVX2 AND/store and fused AND/cardinality/store helpers.
• Added an in-place dense intersection benchmark and differential, aliasing, and dispatch tests.

Why was it changed?

• AVX2-only amd64 CPUs otherwise materialize bitmap intersections with a scalar word loop.
• In-place bitmap intersections need both the written words and their cardinality.

How was it changed?

• Keep the AVX-512 implementation as the first choice, then dispatch to AVX2 when that path is unavailable.
• Load each four-word block before storing it so the helpers support either input as dst; retain scalar tails and generic, appengine, and non-amd64 fallbacks.
• Preserve the existing bitmap-versus-array threshold and cardinality behavior.

Testing

• go test ./...
• go test -tags appengine .
• The AVX2 tests compare vector and scalar tails, empty and short inputs, separate destinations, both aliases, fused cardinality, and dispatch.

Checks: 2 passed.

Performance Impact

The AVX2 fallback processes four words per iteration. The fused helper combines AND, population count, and store for in-place bitmap intersections. The change does not alter allocation or container-representation decisions; unsupported targets keep the existing scalar path.

Workload: dense_bitmap_and_avx2_fallback

Metric	Before	After	Change
ns/op	185176	151533	18.2% lower

Breaking Changes

None.

Additional Notes

This complements the AVX-512 bitmap word operations; AVX-512 remains the preferred path.

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