Fix miscompilation of float vector returns. Compile code to this:

_func:
        vsldoi v2, v3, v2, 12
        vsldoi v2, v2, v2, 4
        blr

instead of:

_func:
        vsldoi v2, v3, v2, 12
        vsldoi v2, v2, v2, 4
***     vor f1, v2, v2
        blr

llvm-svn: 29607