[Mesa-stable] Cherry picks for 12.0 branch (swr + llvm-3.9)

[Rowley, Timothy O]

Cherry picking these three commits from master allow swr on the 12.0 branch to compile/run with llvm 3.6, 3.7, 3.8, and 3.9:

d3d97f8395513bf365d2fe8e4292c8098290586f	swr: [rasterizer jitter] cleanup supporting different llvm versions

ae4f2c849a426c63b149ca5c8f4a293ee5ba751e	swr: [rasterizer jitter] fix llvm-3.7 compile

f8109076696a182da8180b0ee102c5a39f73d36a	swr: [rasterizer] add support for llvm-3.9