[PowerPC] Fix the int2fp(fp2int(x)) DAGCombine to ignore ppc_fp128

This DAGCombine was creating custom SDAG nodes with an illegal ppc_fp128
operand type because it was triggering on f64/f32 int2fp(fp2int(ppc_fp128 x)),
but shouldn't (it should only apply to f32/f64 types). The result was a crash.

llvm-svn: 245530