Running The RadeonSI NIR Back-End With Mesa 19.1 Git

It's been a number of months since last trying the RadeonSI NIR back-end, which is being developed as part of the OpenGL 4.6 SPIR-V bits for this AMD OpenGL driver, but eventually RadeonSI may end up switching from TGSI to NIR by default. Given the time since we last tried it out and the increasing popularity of NIR, this weekend I did some fresh tests of the NIR back-end with a Radeon Vega graphics card.