intel/fs: Emit LINE+MAC for LINTERP with unaligned coordinates

intel/fs: Emit LINE+MAC for LINTERP with unaligned coordinates

On g4x through Sandy Bridge, src1 (the coordinates) of the PLN
instruction is required to be an even register number.  When it's odd
(which can happen with SIMD32), we have to emit a LINE+MAC combination
instead.  Unfortunately, we can't just fall through to the gen4 case
because the input registers are still set up for PLN which lays out the
four src1 registers differently in SIMD16 than LINE.

v2 (Jason Ekstrand):
 - Take advantage of both accumulators and emit LINE LINE MAC MAC
   (Based on a patch from Francisco Jerez)
 - Unify the gen4 and gen4x-6 cases using a loop

v3 (Jason Ekstrand):
 - Don't unify gen4 with gen4x-6 as this turns out to be more fragile
   than first thought without reworking the gen4 barycentric coordinate
   layout.

Reviewed-by: Matt Turner <mattst88@gmail.com>