[SLP] Apply external to vectorizable tree users cost adjustment for

relevant aggregate build instructions only (UserCost).
Users are detected with findBuildAggregate routine and the trick is
that following SLP vectorization may end up vectorizing entire list
with smaller chunks. Cost adjustment then is applied for individual
chunks and these adjustments obviously have to be smaller than the
entire aggregate build cost.

Differential Revision: https://reviews.llvm.org/D80773