radeonsi: drop support for LLVM 3.8

LLVM 3.8:
- had broken indirect resource indexing
- didn't have scratch coalescing
- was the last user of problematic v16i8
- only supported OpenGL 4.1

This leaves us with LLVM 3.9 and LLVM 4.0 support for Mesa 17.2.

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