[PPC] Avoid incorrect fp-i128-fp lowering.

Summary:
Fix an issue that's similar to what D41411 fixed:
  float(__int128(float_var)) shouldn't be optimized to xscvdpsxds +
  xscvsxdsp, as they mean (float)(int64_t)float_var.

Reviewers: jtony, hfinkel, echristo

Subscribers: sanjoy, nemanjai, hiraditya, llvm-commits, kbarton

Differential Revision: https://reviews.llvm.org/D42400

llvm-svn: 323270