split call operands out to their own encoding class, simplifying

code in the JIT.  Use this to form the first fixup for the PPC backend,
giving us stuff like this:

	bl L_foo$stub ; encoding: [0b010010AA,A,A,0bAAAAAA01]
                                        ;   fixup A - offset: 0, value: L_foo$stub, kind: fixup_ppc_br24

llvm-svn: 119123