[PowerPC] Fix PR16556 (handle undef ppcf128 in LowerFP_TO_INT).

PPCTargetLowering::LowerFP_TO_INT() expects its source operand to be
either an f32 or f64, but this is not checked.  A long double
(ppcf128) operand will normally be custom-lowered to a conversion to
f64 in this context.  However, this isn't the case for an UNDEF node.

This patch recognizes a ppcf128 as a legal source operand for
FP_TO_INT only if it's an undef, in which case it creates an undef of
the target type.

At some point we might want to do a wholesale custom lowering of
ISD::UNDEF when the type is ppcf128, but it's not really clear that's
a great idea, and probably more work than it's worth for a situation
that only arises in the case of a programming error.  At this point I
think simple is best.

The test case comes from PR16556, and is a crash-test only.