[SLP] Use getExtractWithExtendCost() to compute the scalar cost of extractelement/ext pair

We use getExtractWithExtendCost to calculate the cost of extractelement and

s|zext together when computing the extract cost after vectorization, but we

calculate the cost of extractelement and s|zext separately when computing the

scalar cost which is larger than it should be.

Differential Revision: https://reviews.llvm.org/D45469