[AMDGPU] Make printf lowering faster when there are no printfs

Summary:
Printf lowering unconditionally visited every instruction in the module.
To make it faster in the common case where there are no printfs, look up
the printf function (if any) and iterate over its users instead.

Reviewers: rampitec, kzhuravl, alex-t, arsenm

Subscribers: jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68145

llvm-svn: 373433