gallium, radeonsi: add a utility to deduplicate live shader CSOs

Creating several shaders with the same IR will return the same CSO, reference-counted.

This happens a lot with games that convert D3D separate shaders to linked GLSL shaders, e.g. the same VS is used with many FS.

The assumed benefits are:

• fewer resident shaders (possibly fewer total shader variants on the driver side)
• fewer shader state changes in the driver if 2 GLSL programs bound back-to-back share shader CSOs

Using a Borderlands 2 apitrace, this is the efficiency with TGSI:

    321 cache hit
    524 cache miss

And with NIR:

    291 cache hit
    554 cache miss

I guess NIR is worse because the linker does more inter-shader optimizations.

radeonsi now has 3 levels of caching:

• live shader CSO deduplication
• in-memory shader cache
• disk shader cache