RFR : PPC64 : Need support for VSR spills in ppc.ad

Hi Martin, Would you review the following change that fixes the SLP for PPC? In this change, I added support for VSR spills. Also, I fixed how to specify registers in postalloc_expand for the float constant replication. I confirmed this change works with JTREG. Bug: https://bugs.openjdk.java.net/browse/JDK-8194861 webrev: http://cr.openjdk.java.net/~mhorie/8194861/webrev.00/ (I created a webrev under jdk/hs.) Best regards, -- Michihiro, IBM Research - Tokyo