i965/fs: Use NIR's scalarizing abilities and stop handling vectors

i965/fs: Use NIR's scalarizing abilities and stop handling vectors

Now that we can scalarize with NIR, there's no need for all this code
anymore.  Let's get rid of it and just do scalar operations.

v2: run copy prop before lowering phi nodes

v3: Get rid of the "emit(...)->saturate = foo" pattern

v4: Run alu_to_scalar as an optimization pass

total instructions in shared programs: 5998321 -> 5974070 (-0.40%)
instructions in affected programs:     732075 -> 707824 (-3.31%)
helped:                                3137
HURT:                                  191
GAINED:                                18
LOST:                                  0

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>