PPC64: Mapping floating point registers to vsx registers in ppc.ad

In some code emit for replicating the floating point value in ppc.ad, a floating point value is once stored in order to load as an integer value. When SuperwordUseVSX is enabled, this is redundant because the floating point registers are mapped to vsx registers 0-31. We can use vsx instructions for replicating the floating point value.