i965/vec4: Add parameter to skip doing constant propagation.

i965/vec4: Add parameter to skip doing constant propagation.

After CSEing some MOV ..., VF instructions we have code like

   mov tmp, [1F, 2F, 3F, 4F]VF
   mov r10, tmp
   mov r11, tmp
   ...
   use r10
   use r11

We want to copy propagate tmp into the uses of r10 and r11, but *not*
constant propagate the VF immediate into the uses of tmp.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>