[SLP] Support vectorizing functions provided by vector libs.

It seems like the SLPVectorizer is currently not aware of vector
versions of functions provided by libraries like Accelerate [1].
This patch updates SLPVectorizer to use the same infrastructure
the LoopVectorizer uses to detect vectorizable library functions.

For calls, it computes the cost of an intrinsic call (existing behavior)
and the cost of a vector function library call, if available. Like
LoopVectorizer, it assumes the cost of the vector function is simply the
cost of a call to a vector function.

[1] https://developer.apple.com/documentation/accelerate

Reviewers: ABataev, RKSimon, spatel

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D75878