Power9] Implement new vsx instructions: compare and conversion

This change implements the following vsx instructions:

Quad/Double-Precision Compare:
xscmpoqp xscmpuqp
xscmpexpdp xscmpexpqp
xscmpeqdp xscmpgedp xscmpgtdp xscmpnedp
xvcmpnedp(.) xvcmpnesp(.)
Quad-Precision Floating-Point Conversion
xscvqpdp(o) xscvdpqp
xscvqpsdz xscvqpswz xscvqpudz xscvqpuwz xscvsdqp xscvudqp
xscvdphp xscvhpdp xvcvhpsp xvcvsphp
xsrqpi xsrqpix xsrqpxp
28 instructions

Phabricator: http://reviews.llvm.org/D16709
llvm-svn: 262068