Update Eigen to commit:8a9bfb72d7074b63d8e6af12f78ef2fa0c61f4e3

CHANGELOG
=========
8a9bfb72d - Rename preduce_half for HVX.
ed989c750 - Enable generic clang backend tests.
3368ac6c6 - Don't set platform-specific vectorization macros for generic backend.
fecfa7f27 - Fixes to make generic backend build with AVX512
ec93a6d09 - Add a generic Eigen backend based on clang vector extensions

PiperOrigin-RevId: 829512412
Change-Id: I088d23927b2ebdf35dd337f5187a1070464d2fb0
22 files changed