[SLP]Generalize cost model.

Generalized the cost model estimation. Improved cost model estimation
for repeated scalars (no need to count their cost anymore), improved
  cost model for extractelement instructions.

cpu2017
   511.povray_r             0.57
   520.omnetpp_r           -0.98
   521.wrf_r               -0.01
   525.x264_r               3.59 <+
   526.blender_r           -0.12
   531.deepsjeng_r         -0.07
   538.imagick_r           -1.42
Geometric mean:  0.21

Differential Revision: https://reviews.llvm.org/D115757