movc3 is NOT always faster!

[Dave Martindale]

Movc[35] normally does 32-bit writes to memory, at least on the 780.
Reads are always 64 bits due to the cache.  The actual data transfer
is slower than doing a movq, since 4 SBI cycles are required for 2 32-bit
writes vs. 3 for one 64-bit write.  Also, if you have the old MS780C
memory controller, a 32-bit write has to do a read-modify-write cycle
for any write smaller than 64 bits.  The new controllers don't have
this problem.