Connor previously proposed the "Flatland" GLSL IR and the work went on to other Mesa IR discussions while the past few months he's been paid by Intel to work out a new Mesa IR. This new IR is called "NIR" and aims to be lower-level than GLSL IR but still high enough to be device-independent and support generic optimizations. NIR is flat, type-less, GLSL-like features, native support for SSA, and uses much less memory than GLSL IR.
On Friday there was the public announcement for this new IR for Mesa while it's still considered experimental and not yet complete. When it comes to Mesa drivers actually supporting it, there's an experimental back-end for the Intel i965 classic driver but it's not yet in a state for merging and currently the NIR code for Mesa still needs to take GLSL IR before translating it to NIR. Originally the student developer was planning to just rework GLSL IR until realizing its faults were too many and determined it would be better in the long run to just develop NIR.
Those wishing to learn more about this new IR for Mesa can find out more information via the Mesa announcement.