i965/vec4: teach register coalescing about 64-bit

i965/vec4: teach register coalescing about 64-bit

Specifically, at least for now, we don't want to deal with the fact that
channel sizes for fp64 instructions are twice the size, so prevent
coalescing from instructions with a different type size.

Also, we should check that if we are coalescing a register from another
MOV we should be writing the same amount of data in both operations, otherwise
we end up wiring more or less than the original instruction. This can happen,
for example, when we have split fp64 MOVs with an exec size of 4 that only
write one register each and then a MOV with exec size of 8 that reads both.
We want to avoid the pass to think that it can coalesce from the first split
MOV alone. Ideally we would like the pass to see that it can coalesce from both
split MOVs instead, but for now we keep it simple.

Finally, the pass doesn't support coalescing of multiple registers but in the
case of normal SIMD4x2 double-precision instructions they naturally write two
registers (one per vertex) and there is no reason why we should not allow
coalescing in this case. Change the restriction to bail if we see instructions
that write more than 8 channels, where the channels can be 32-bit or 64-bit.

v2:
 - Make sure that scan_inst and inst write the same amount of data.

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