[Inlining] Make shouldBeDeferred static (NFC)

Summary:
This patch makes shouldBeDeferred static because it is called only
from shouldInline in the same .cpp file.

Reviewers: davidxl, mtrofin

Reviewed By: mtrofin

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79750