[MicroBenchmarks] Add initial SLP vectorization benchmarks.

This patch adds a first set of micro benchmarks for SLP vectorization.
The initial benchmarks are focused on cases that require memory
versioning (worked on in D102834).

D101844 added similar benchmarks for the loop vectorizer.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D104126
4 files changed