Fix darwin ppc llvm-gcc build breakage: intercept

ppcf128 to i32 conversion and expand it into a code
sequence like in LegalizeDAG.  This needs custom
ppc lowering of FP_ROUND_INREG, so turn that on and
make it work with LegalizeTypes.  Probably PPC should
simply custom lower the original conversion.

llvm-svn: 58329