[SLP] Take user instructions cost into consideration in insertelement vectorization.

Summary:
For better vectorization result we should take into consideration the
cost of the user insertelement instructions when we try to
vectorize sequences that build the whole vector. I.e. if we have the
following scalar code:
```
<Scalar code>
insertelement <ScalarCode>, ...
```
we should consider the cost of the last `insertelement ` instructions as
the cost of the scalar code.

Reviewers: RKSimon, spatel, hfinkel, mkuper

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D42657

llvm-svn: 324893