i965/vec4: implement HW workaround for align16 double to float conversion

i965/vec4: implement HW workaround for align16 double to float conversion

From the BDW PRM, Workarounds chapter:

   "DF->f format conversion for Align16 has wrong emask calculation when
    source is immediate."

Notice that Broadwell and later are strictly scalar at the moment though, so
this is not really necessary.

v2: Instead of moving the immediate to a vgrf and converting from there, just
    convert the double immediate to float in the compiler and move the result
    to the destination (Matt)

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