be: Simplify check in prepare_constr_insn() a bit.
be: Simplify check in prepare_constr_insn() a bit.

Test that the inputs are identical first.
If this is the case, then the register classes must be identical and therefore need not be tested.

libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm