AMDGPU: Disable AMDGPUPromoteAlloca pass for shader calling conventions.

Summary:
The work item intrinsics are not available for the shader
calling conventions. And even if we did hook them up most
shader stages haves some extra restrictions on the amount
of available LDS.

Reviewers: tstellarAMD, arsenm

Subscribers: nhaehnle, arsenm, llvm-commits, kzhuravl

Differential Revision: https://reviews.llvm.org/D20728

llvm-svn: 275779