AMDGPU: Add amdgpu-ps-wqm-outputs function attributes

Summary:
The presence of this attribute indicates that VGPR outputs should be computed
in whole quad mode. This will be used by Mesa for prolog pixel shaders, so
that derivatives can be taken of shader inputs computed by the prolog, fixing
a bug.

The generated code could certainly be improved: if a prolog pixel shader is
used (which isn't common in modern OpenGL - they're used for gl_Color, polygon
stipples, and forcing per-sample interpolation), Mesa will use this attribute
unconditionally, because it has to be conservative. So WQM may be used in the
prolog when it isn't really needed, and furthermore a silly back-and-forth
switch is likely to happen at the boundary between prolog and main shader
parts.

Fixing this is a bit involved: we'd first have to add a mechanism by which
LLVM writes the WQM-related input requirements to the main shader part binary,
and then Mesa specializes the prolog part accordingly. At that point, we may
as well just compile a monolithic shader...

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95130

Reviewers: arsenm, tstellarAMD, mareko

Subscribers: arsenm, llvm-commits, kzhuravl

Differential Revision: http://reviews.llvm.org/D20839

llvm-svn: 272063