Clean up register constraints.

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/CIHBJEHG.html
 says that "Rd cannot be the same as Rm."
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0068b/CIHBJEHG.html
 says that "RdLo, RdHi, and Rm must all be different registers."
This means that some of the early clobbers I removed really should
 have been there (to prevent aliasing Rd, RdLo, or RdHi with Rm).
It also means that we should reverse some of the operands in the
 FFT's complex multiplies.
This should only affect the ARMv4 optimizations.

Thanks to Nils Wallménius for the report.

While we're here, audit the commutative pair flags again, since I
 screwed up at least one of them, and eliminate some dead code.