i965/fs: Fix compute-to-mrf VGRF region coverage condition.

i965/fs: Fix compute-to-mrf VGRF region coverage condition.

Compute-to-mrf was checking whether the destination of scan_inst is
more than one component (making assumptions about the instruction data
type) in order to find out whether the result is being fully copied
into the MRF destination, which is rather inaccurate in cases where a
single-component instruction is only partially contained in the source
region, or when the execution size of the copy and scan_inst
instructions differ.  Instead check whether the destination region of
the instruction is really contained within the bounds of the source
region of the copy.

Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>