Remove and simplify some of the bewildering collection of isFOORegister

methods which have strangely different semantics in different backends,
and noone knew what any did.

Getting rid of these ALSO allows the dependence of MachineInstr.h on
MRegisterInfo.h to be removed, which makes me much happier, and probably
alkis too.  :)

llvm-svn: 11287