radeonsi: eliminate VS outputs that aren't used by PS at runtime

radeonsi: eliminate VS outputs that aren't used by PS at runtime

A past commit added the ability to compile "optimized" shader variants
asynchronously (not stalling the app).

This commit builds upon that and adds what is basically a runtime shader
linker. If a VS output isn't used by the currently-bound PS, a new VS
compilation is started without that output. The new shader variant
is used when it's ready.

All apps using separate shader objects I've seen had unused VS outputs.

Eliminating unused/useless VS outputs also eliminates the corresponding
vertex attribute loads.

Tested-by: Edmondo Tommasina <edmondo.tommasina@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>