Deprecate pre-GFX10 support in LLPC about llpc

[Triang3l]

And I was thinking about using LLVM on GFX4/GFX5, but I couldn't even find the intrinsic for vertex fetch (let alone one with ALT_CONST so I could share the binding for reading from the ring buffer between GS and the "GS copy VS" while leaving one precious ES/VS fetch constant for indirect GL_ARB_shader_draw_parameters in the real VS, though that's not really necessary if the copy shader is generated by other means)…

[Flakebi]

The removal of pre-GFX10 affects LLPC, as it is tightly coupled to PAL. It does not affect LLVM. LLVM keeps its existing support for radeonsi and others.

[amdrexu]

Yes, the removal of pre-GFX10 support will only affect AMDVLK. LLVM will still keep the codes that support old generations (nothing will be removed).