[TargetTransformInfo] assert on nullptr
[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

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@362567 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm