i965/fs: Fix split_virtual_grfs() when delta_xy not in a virtual register.

i965/fs: Fix split_virtual_grfs() when delta_xy not in a virtual register.

This patch modifies the special case in
fs_visitor::split_virtual_grfs() that prevents splitting from being
applied to the delta_x/delta_y register pair (this register pair needs
to remain contiguous so that it can be used by the PLN instruction).

When gen>=6, this register pair is in a fixed location, not a virtual
register, so it was in no danger of being split.  And
split_virtual_grfs' attempt not to split it was preventing some other
unrelated register from being split.

Reviewed-by: Eric Anholt <eric@anholt.net>