[SLP]Extend vectorization for scatter vectorize nodes.

Currently scatter vectorize nodes can be emitted only for GEPs with
constant indices. But we can also emit such nodes for GEPs with the same
ptr and non-constant vectorizable/gathered indices, if profitable. Patch
adds support for such nodes and tries to improve handling of GEPs with
non-const indeces for such nodes.

Metric: SLP.NumVectorInstructions

Program                                                                                       SLP.NumVectorInstructions
                                                                                              results                   results0 diff
                    test-suite :: External/SPEC/CFP2017speed/638.imagick_s/638.imagick_s.test  5243.00                   5240.00  -0.1%
                     test-suite :: External/SPEC/CFP2017rate/538.imagick_r/538.imagick_r.test  5243.00                   5240.00  -0.1%
                     test-suite :: External/SPEC/CFP2017rate/526.blender_r/526.blender_r.test 27550.00                  27507.00  -0.2%
                               test-suite :: External/SPEC/CFP2006/453.povray/453.povray.test  5395.00                   5380.00  -0.3%
                       test-suite :: External/SPEC/CFP2017rate/511.povray_r/511.povray_r.test  5389.00                   5374.00  -0.3%
                    test-suite :: External/SPEC/CINT2017rate/520.omnetpp_r/520.omnetpp_r.test   961.00                    958.00  -0.3%
                   test-suite :: External/SPEC/CINT2017speed/620.omnetpp_s/620.omnetpp_s.test   961.00                    958.00  -0.3%
                               test-suite :: External/SPEC/CFP2006/447.dealII/447.dealII.test  5664.00                   5643.00  -0.4%
                       test-suite :: External/SPEC/CFP2017rate/510.parest_r/510.parest_r.test 13202.00                  13127.00  -0.6%
                                test-suite :: External/SPEC/CINT2006/445.gobmk/445.gobmk.test   212.00                    207.00  -2.4%
                                test-suite :: MultiSource/Benchmarks/7zip/7zip-benchmark.test   890.00                    850.00  -4.5%
                            test-suite :: External/SPEC/CINT2006/464.h264ref/464.h264ref.test  1695.00                   1581.00  -6.7%
                                 test-suite :: MultiSource/Applications/JM/lencod/lencod.test  2338.00                   2140.00  -8.5%
                                  test-suite :: SingleSource/UnitTests/matrix-types-spec.test    63.00                     55.00 -12.7%
                             test-suite :: SingleSource/Benchmarks/Adobe-C++/loop_unroll.test   468.00                    356.00 -23.9%
                                                                           Geomean difference                                     -0.3%

All numbers show increased number of generated vector instructions.

Diff:
SingleSource/Benchmarks/Adobe-C++/loop_unroll - better without LTO, but
need an extra analysis with LTO (with LTO compiler generates
masked_gather, while before regular loads were emitted because of extra
data, availbale at LTO time).
SingleSource/UnitTests/matrix-types-spec - more vector code.
MultiSource/Applications/JM/lencod/lencod - same.
External/SPEC/CINT2006/464.h264ref/464.h264ref - same.
MultiSource/Benchmarks/7zip/7zip-benchmark - same.
External/SPEC/CINT2006/445.gobmk/445.gobmk - no changes.
External/SPEC/CFP2017rate/510.parest_r/510.parest_r - more vector code.
External/SPEC/CFP2006/447.dealII/447.dealII - same
External/SPEC/CINT2017speed/620.omnetpp_s/620.omnetpp_s - same
External/SPEC/CINT2017rate/520.omnetpp_r/520.omnetpp - same
External/SPEC/CFP2017rate/511.povray_r/511.povray - same
External/SPEC/CFP2006/453.povray/453.povray - same
External/SPEC/CFP2017rate/526.blender_r/526.blender_r - same
External/SPEC/CFP2017rate/538.imagick_r/538.imagick_r - same
External/SPEC/CFP2017speed/638.imagick_s/638.imagick_s - same

Differential Revision: https://reviews.llvm.org/D127219