[AMDGPU][NFC] Simplify test file for amdgcn intrinsics

Summary: Remove unchecked attribute in the call site and use FileCheck String Substitution for `convergent` check.

Reviewers: nhaehnle

Reviewed By: nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64901

llvm-svn: 366781