i965/fs: Less broken handling of force_writemask_all in lower_load_payload().

i965/fs: Less broken handling of force_writemask_all in lower_load_payload().

It's perfectly fine to read the second half of a register written with
force_writemask_all from a first half MOV instruction or vice versa, and
lower_load_payload shouldn't mark the whole MOV as belonging to the second
half in that case.  Replicate the same metadata to both halves of the
destination when writemasking is disabled.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>