[AMDGPU] Add support for immediate operand for S_ENDPGM

Summary:
Add support for immediate operand in S_ENDPGM

Change-Id: I0c56a076a10980f719fb2a8f16407e9c301013f6

Reviewers: alexshap

Subscribers: qcolombet, arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, eraman, arphaman, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59213

llvm-svn: 355902