[PATCH] [PowerPC] Fix unwind info with dynamic stack realignment

[Jay Foad]

Incidentally, is there any need to have accurate unwind info *before* we copy SP to FP? If not, we could do it all after the copy to FP, simplifying the code a bit and probably saving the odd byte in the CFI encoding:
```
  if (HasBP)
    .cfi_def_cfa_register BP
  else if (HasFP)
    .cfi_def_cfa FP, NegFrameSize
  else
    .cfi_def_cfa_offset NegFrameSize
```

http://reviews.llvm.org/D6410