[SLP]Keep externally used GEPs as GEPs, if possible instead of extractelement.

If the vectorized GEP instruction can be still kept as a scalar GEP,
better to keep it as scalar instead of extractelement. In many cases it
is more profitable.

Metric: size..text

Program                                                                          size..text
                                                                                 results     results0    diff
                        test-suite :: SingleSource/Benchmarks/Misc/oourafft.test    18911.00    19695.00  4.1%
                   test-suite :: SingleSource/Benchmarks/Misc-C++-EH/spirit.test    59987.00    60707.00  1.2%
       test-suite :: External/SPEC/CFP2017speed/638.imagick_s/638.imagick_s.test  1392209.00  1392753.00  0.0%
        test-suite :: External/SPEC/CFP2017rate/538.imagick_r/538.imagick_r.test  1392209.00  1392753.00  0.0%
           test-suite :: External/SPEC/CINT2006/400.perlbench/400.perlbench.test  1087996.00  1088236.00  0.0%
                         test-suite :: MultiSource/Benchmarks/Bullet/bullet.test   309310.00   309342.00  0.0%
             test-suite :: External/SPEC/CINT2017rate/525.x264_r/525.x264_r.test   664661.00   664693.00  0.0%
            test-suite :: External/SPEC/CINT2017speed/625.x264_s/625.x264_s.test   664661.00   664693.00  0.0%
        test-suite :: External/SPEC/CFP2017rate/526.blender_r/526.blender_r.test 12354636.00 12354908.00  0.0%
                  test-suite :: External/SPEC/CFP2006/453.povray/453.povray.test  1152748.00  1152716.00 -0.0%
                       test-suite :: MultiSource/Applications/oggenc/oggenc.test   191787.00   191771.00 -0.0%
                     test-suite :: SingleSource/UnitTests/matrix-types-spec.test   480796.00   480476.00 -0.1%

Misc/oourafft - Extra code gets vectorized
Misc-C++-EH/spirit - same
CFP2017speed/638.imagick_s
CFP2017rate/538.imagick_r - same, extra code gets vectorized
CINT2006/400.perlbench - some extra 4 x ptr stores vectorized
Bullet/bullet - extra 4 x ptr store vectorized
CINT2017rate/525.x264_r
CINT2017speed/625.x264_s - same
CFP2017rate/526.blender_r - extra 8 x float stores (several), some extra
4 x ptr stores
CFP2006/453.povray - 2 x double loads/stores replaced by 4 x double
loads/stores
Applications/oggenc - extra code is vectorized
UnitTests/matrix-types-spec - extra code gets vectorized

Reviewers: RKSimon

Reviewed By: RKSimon

Pull Request: https://github.com/llvm/llvm-project/pull/88877