i965/fs: Add do_untyped_vector_read helper

i965/fs: Add do_untyped_vector_read helper

We are going to need the same logic for anything that reads
doubles via untyped messages (CS shared variables and SSBOs). Add a
helper function with that logic so that we can reuse it.

v2:
- Make this a static function instead of a method of fs_visitor (Iago)
- We only support types with a size of 4 or 8 (Curro)
- Avoid retypes by using a separate vgrf for the packed result (Curro)
- Put dst parameter before source parameters (Curro)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>