[PowerPC] Emit xscpsgndp instead of xxlor when copying floating point scalar registers for P9

This patch will address using the xscpsgndp instruction to copy floating point
scalar registers instead of the xxlor (specifically XXLORf) instruction that is
currently used. Additionally, this patch of utilizing xscpsgndp will apply to
P9, while pre-P9 will still use xxlor.

Patch by amyk

Differential Revision: https://reviews.llvm.org/D50004

llvm-svn: 340643