i965/fs: Add support for specifying register horizontal strides.

i965/fs: Add support for specifying register horizontal strides.

v2: Some improvements for copy propagation with non-contiguous
    register strides and mismatching types.
v3: Add example of the situation that the copy propagation changes are
    intended to avoid.  Clarify that 'fs_reg::apply_stride()' is expected
    to work with zero strides too.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>