[TargetTransformInfo] assert on nullptr

Summary:
This was flagged in https://www.viva64.com/en/b/0629/ under "Snippet No.
38".

Add an assertion, since it's unlikely that this parameter is nullptr.

Reviewers: RKSimon, fhahn

Reviewed By: RKSimon

Subscribers: fhahn, llvm-commits, RKSimon, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62229

llvm-svn: 362567