i965/vec4: support basic spilling of 64-bit registers

i965/vec4: support basic spilling of 64-bit registers

The current spilling code can't spill vgrf allocations larger than 1
but SIMD4x2 doubles require 2 vgrfs, so we need to permit this case (which
is handled properly for DF data types by emitting 2 scratch messages and
doing data shuffling). We accomplish this by not auto-disabling spilling
for vgrf allocations with a size of 2, and then disable spilling on any
register with an offset != 0B (which indicates array access).

Disable spilling of partial DF reads/writes because these don't read/write
data for both logical threads and our scratch messages for 64-bit data
need data for both threads to be present.

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