freedreno/ir3: add NIR compiler

freedreno/ir3: add NIR compiler

The NIR compiler frontend is an alternative to the TGSI f/e, producing
the same ir3 IR and using the same backend passes for scheduling, etc.

It is not enabled by default yet, as there are still some regressions.
To enable, use 'FD_MESA_DEBUG=nir'.  It is enough to use with, for
example, xonotic or supertuxkart.

With the NIR f/e, scalarizing and a number of other lowering steps
happen in NIR, so we don't have to do them in ir3.  Which simplifies the
f/e and allows the lowered instructions to pass through other
optimization stages.

Signed-off-by: Rob Clark <robclark@freedesktop.org>