Prefer to allocate V2-V5 before V0,V1. This lets us generate code like this:

        vspltisw v2, -12
        vrlw v2, v2, v2

instead of:

        vspltisw v0, -12
        vrlw v2, v0, v0

when a function is returning a value.

llvm-svn: 27771