belower: if use_copies is set, also convert transpositions to copies
belower: if use_copies is set, also convert transpositions to copies

If use_copies is set (i.e. if we prefer copies over exchanges) and a free
register is available, we want to lower transpositions to sets of three
copies.

This improves performance on x86, since mov is essentially free but xchg
takes 3 uops.

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