[SLP]Fix cost estimation for buildvectors with extracts and/or constants.

If the partial matching is found and some other scalars must be
inserted, need to account the cost of the extractelements, transformed
to shuffles, and/or reused entries and calculate the cost of inserting
constants properly into the non-poison vectors.
Also, fixed the cost calculation for final gather/buildvector sequence.

Differential Revision: https://reviews.llvm.org/D148362