[SLP] Enhance SLPVectorizer to vectorize vector aggregate

Summary:
Vector aggregate is homogeneous aggregate of vectors like `{ <2 x float>, <2 x float> }`.
This patch allows `findBuildAggregate()` to consider vector aggregates as
well as scalar ones. For instance, `{ <2 x float>, <2 x float> }` maps to `<4 x float>`.

Fixes vector part of llvm.org/PR42022

Reviewers: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70068