[PowerPC] Do not legalize vector FDIV without VSX

Quite a while ago, we legalized these nodes as we added custom
handling for reciprocal estimates in the back end. We have since
moved to target-independent combines but neglected to turn off
legalization. As a result, we can now get selection failures on
non-VSX subtargets as evidenced in the listed PR.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=47373