Continue Alkis's int64_t cleanup. This makes all of the immediate related

methods take an int or unsigned value instead of int64_t.

Also, add an 'addImm' method to the MachineInstrBuilder class, because the
fact that the hardware sign or zero extends it does not/should not matter
to the code generator.  Once the old sparc backend is removed the difference
can be eliminated.

llvm-svn: 11976