freedreno: add tgsi lowering pass

freedreno: add tgsi lowering pass

Currently lowers the following instructions:

   DST, XPD, SCS, LRP, FRC, POW, LIT, EXP, LOG, DP4,
   DP3, DPH, DP2

translating these into equivalent simpler TGSI instructions.

This probably should be moved to util so other drivers can use
it, but just adding under freedreno for now so that I can clear
out a lot of the lowering code in a3xx compiler before beginning
to add new compiler.

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