i965/vec4: Avoid reswizzling MACH instructions in opt_register_coalesce().

i965/vec4: Avoid reswizzling MACH instructions in opt_register_coalesce().

opt_register_coalesce() was optimizing sequences such as:

   mul(8) acc0:D, attr18.xyyy:D, attr19.xyyy:D
   mach(8) vgrf5.xy:D, attr18.xyyy:D, attr19.xyyy:D
   mov(8) m4.zw:F, vgrf5.xxxy:F

into:

   mul(8) acc0:D, attr18.xyyy:D, attr19.xyyy:D
   mach(8) m4.zw:D, attr18.xxxy:D, attr19.xxxy:D

This doesn't work - if we're going to reswizzle MACH, we'd need to
reswizzle the MUL as well.  Here, the MUL fills the accumulator's .zw
components with attr18.yy * attr19.yy.  But the MACH instruction expects
.z to contain attr18.x * attr19.x.  Bogus results ensue.

No change in shader-db on Haswell.  Prevents regressions in Timothy's
patches to use enhanced layouts for varying packing (which rearrange
code just enough to trigger this pre-existing bug, but were fine
themselves).

Acked-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>