[PowerPC] Don't attempt a 64-bit pow2 division on PPC32

In r224033, in moving the signed power-of-2 division expansion into
BuildSDIVPow2, I accidentally made it possible to attempt the lowering for a
64-bit division on PPC32. This later asserts.

Fixes PR21928.

llvm-svn: 224758
diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 62c1f71..1838462 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -8966,6 +8966,8 @@
                                   std::vector<SDNode *> *Created) const {
   // fold (sdiv X, pow2)
   EVT VT = N->getValueType(0);
+  if (VT == MVT::i64 && !Subtarget.isPPC64())
+    return SDValue();
   if ((VT != MVT::i32 && VT != MVT::i64) ||
       !(Divisor.isPowerOf2() || (-Divisor).isPowerOf2()))
     return SDValue();