Fix a couple of the FIXMEs, thanks to suggestion from Chris. This allows

us to load and store vectors directly at a pointer (offset of zero) by
using r0 as the base register.  This also requires some asm printer work
to satisfy the darwin assembler.

For
void %foo(<4 x float> * %a) {
entry:
  %tmp1 = load <4 x float> * %a;
  %tmp2 = add <4 x float> %tmp1, %tmp1
  store <4 x float> %tmp2, <4 x float> *%a
  ret void
}

We now produce:
_foo:
        lvx v0, 0, r3
        vaddfp v0, v0, v0
        stvx v0, 0, r3
        blr

Instead of:
_foo:
        li r2, 0
        lvx v0, r2, r3
        vaddfp v0, v0, v0
        stvx v0, r2, r3
        blr

llvm-svn: 24872