i965/vec4: Don't spill non-GRF-aligned register regions.

i965/vec4: Don't spill non-GRF-aligned register regions.

A better fix would be to do something along the lines of the FS
back-end spilling code and emit a scratch read before any instruction
that overwrites the register to spill partially due to a non-zero
sub-register offset.  In the meantime mark registers used with a
non-zero sub-register offset as no-spill to prevent the spilling code
from miscompiling the program.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>