[SLP] Enhance SLPVectorizer to vectorize different combinations of aggregates

Summary:
Make SLPVectorize to recognize homogeneous aggregates like
`{<2 x float>, <2 x float>}`, `{{float, float}, {float, float}}`,
`[2 x {float, float}]` and so on.
It's a follow-up of https://reviews.llvm.org/D70068.
Merged `findBuildVector()` and `findBuildAggregate()` to
one `findBuildAggregate()` function making it recursive
to recognize multidimensional aggregates. Aggregates required
to be homogeneous.

Reviewers: RKSimon, ABataev, dtemirbulatov, spatel, vporpo

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70587