intel/brw: Fixes and VEC preparation patches from !28666
This is a subset of the patches from !28666 with the bug fixes and VEC prep patches. One change is that the commit to add the VEC helper still does the MOV
loop in this rendition, rather than a LOAD_PAYLOAD
. That way, we can land a lot of the churn to use the helper without changing what gets generated. Then we can swap the helper over in an updated !28666.