PPC: Mark vector FREM as Expand by default

Another bug found by llvm-stress! This fixes crashing with:
  LLVM ERROR: Cannot select: v4f32 = frem ...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185840 91177308-0d34-0410-b5e6-96231b3b80d8