gallium: add TGSI property NEXT_SHADER

Radeonsi needs to know which shader stage will execute after a shader
in order to make the best decision about which shader variant to compile
first.

This is only set for VS and TES, because we don't need it elsewhere.

VS has 3 variants:
- next shader is FS
- next shader is GS
- next shader is TCS

TES has 2 variants:
- next shader is FS
- next shader is GS

Currently, radeonsi always assumes the next shader is FS, which is suboptimal,
since st/mesa always knows which shader is next if the GLSL program is not
a "separate shader".

By default, ureg always sets "next shader is FS".

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/gallium/docs/source/tgsi.rst

@@ -3213,6 +3213,14 @@ Whether depth test, stencil test, and occlusion query should run before
 the fragment shader (regardless of fragment shader side effects). Corresponds
 to GLSL early_fragment_tests.
 
+NEXT_SHADER
+"""""""""""
+
+Which shader stage will MOST LIKELY follow after this shader when the shader
+is bound. This is only a hint to the driver and doesn't have to be precise.
+Only set for VS and TES.
+
+
 Texture Sampling and Texture Formats
 ------------------------------------