i965/fs: Rewrite fs_visitor::split_virtual_grfs

i965/fs: Rewrite fs_visitor::split_virtual_grfs

The original vgrf splitting code was written with the assumption that vgrfs
came in two types: those that can be split into single registers and those
that can't be split at all It was very conservative and bailed as soon as
more than one element of a register was read or written.  This won't work
once we start allowing a regular MOV or ADD operation to operate on
multiple registers.  This rewrite allows for the case where a vgrf of size
5 may appropriately be split in to one register of size 1 and two registers
of size 2.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Acked-by: Matt Turner <mattst88@gmail.com>