[release/6.0] Fix incorrect SIMD temp allocation for Vector256 with AVX2 disabled#58850

jeffschwMSFT merged 2 commits intorelease/6.0dotnet/runtime:release/6.0from

backport/pr-58820-to-release/6.0dotnet/runtime:backport/pr-58820-to-release/6.0Copy head branch name to clipboard