[PowerPC] Fix parest build failure in SPEC2017.

The build failure was caused by an assertion in pre-legalization DAGCombine:

Combining: t6: ppcf128 = uint_to_fp t5
... into: t20: f32 = PPCISD::FCFIDUS t19

which is clearly wrong since ppcf128 are definitely different type with f32 and
we cannot change the node value type when do DAGCombine. The fix is don't
handle ppc_fp128 or i1 conversions in PPCTargetLowering::combineFPToIntToFP and
leave it to downstream to legalize it and expand it to small legal types.

Differential Revision: https://reviews.llvm.org/D41411

llvm-svn: 321276