Optimize FSEL a bit for fneg arguments. This fixes the recently added test

case so that we emit

_test_fneg_sel:
.LBB_test_fneg_sel_0:   ;
        fsel f1, f1, f3, f2
        blr

instead of:

_test_fneg_sel:
.LBB_test_fneg_sel_0:   ;
        fneg f0, f1
        fneg f0, f0
        fsel f1, f0, f3, f2
        blr

llvm-svn: 21177