[SLP]Improve isUndefVector function by adding insertelement analysis.

Added the mask and the analysis of the buildvector sequence in the
isUndefVector function, improves codegen and cost estimation.

Metric: SLP.NumVectorInstructions

Program                                                                                       SLP.NumVectorInstructions
                                                                          results                   results0 diff
test-suite :: External/SPEC/CFP2017rate/526.blender_r/526.blender_r.test 27362.00                  27360.00 -0.0%

Metric: size..text

Program                                                                                                           size..text
                                                                   results     results0    diff
test-suite :: External/SPEC/CFP2017rate/508.namd_r/508.namd_r.test   805299.00   806035.00  0.1%

526.blender_r - some extra code is vectorized.
508.namd_r - some extra code is optimized out.

Differential Revision: https://reviews.llvm.org/D133891