Since pack_bytes expands to two mov(4) align1 instructions, we can't use
swizzles directly. For an instruction like

   pack_bytes m4.y:UD, vgrf13.xyzw:UD

we can write into the .y component by settings the offset based on the
swizzle.

Also while we're doing this, we can set the dependency control hints
properly, so that a series of pack_bytes writing into separate
components of a register can issue without blocking.