(acc and ax) and product register with one read/write.
Gives a minuscule speedup of not more than 4%. In exchange, breaks all
your out-of-tree changes to dsp. Tests are not building again, yet.
git-svn-id: https://dolphin-emu.googlecode.com/svn/trunk@6680 8ced0084-cf51-0410-be5f-012b33b47a6e