[SLPVectorizer] handle vectorizeable library functions

Teaches the SLPVectorizer to use vectorized library functions for
non-intrinsic calls.

This already worked for intrinsics that have vectorized library
functions, thanks to D75878, but schedules with library functions with a
vector variant were being rejected early.

-   assume that there are no load/store dependencies between lib
    functions with a vector variant; this would otherwise prevent the
    bundle from becoming "ready"

-   check during legalization that the vector variant can be used

-   fix-up where we previously assumed that a call would be an intrinsic

Differential Revision: https://reviews.llvm.org/D82550