[cherrypick] Unroll loop in lookup_2_lanes for PQFasfScan for PPC64

[alexanderguzhva]

Cherry pick commit facebookresearch/faiss#3364 from faiss baseline

This change results in approximately a 55% reduction in the execution time for the bench_ivf_fastscan.py workload on Power 10 when compiled with CMAKE_INSTALL_CONFIG_NAME=Release.

@carll99 fyi

[mergify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.14%. Comparing base (3c46f4c) to head (a42b9a6).
Report is 22 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##           main     #524       +/-   ##
=========================================
+ Coverage      0   71.14%   +71.14%     
=========================================
  Files         0       67       +67     
  Lines         0     4401     +4401     
=========================================
+ Hits          0     3131     +3131     
- Misses        0     1270     +1270     

see 67 files with indirect coverage changes

[liliu-z]

/lgtm

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[liliu-z]

/kind improvement