i965/fs: retype offset_reg to UD at load_ssbo
All operations with offset_reg at do_vector_read are done
with UD type. So copy propagation was not working through
the generated MOVs:
mov(8) vgrf9:UD, vgrf7:D
This change allows removing the MOV generated for reading the
first components for 16-bit and 64-bit ssbo reads with
non-constant offsets.
Reviewed-by:
Iago Toral Quiroga <itoral@igalia.com>