[SamplePGO] Minor efficiency improvement in samplePGO ICP

Summary:
When attaching prof metadata to promoted direct calls in SamplePGO
mode, no need to construct and use a SmallVector to pass a single count
to the ArrayRef parameter, we can simply use a brace-enclosed init list.

This made a small but consistent improvement for a ThinLTO backend
compile I was measuring.

Reviewers: wmi

Subscribers: mehdi_amini, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57706

llvm-svn: 353123