backporting extract_bit_field patch to 3.1

[Aldy Hernandez]

i'm not going to push this too much, but...

> Unless you can find a regression on non-Altivec code, that is...

sure...replace vec_xor for whatever the SSE variant is, define

	#define vector __attribute__((vector_size(N)))

	where N is the number of bytes in say, SSE SIMD registers and
	voila... it's a problem with the SIMD stuff, not necessarily
	altivec.

aldy

vector signed short b16;
vector signed short poop;

int
main ()
{
  vector signed short hannah = poop;

  b16 = vec_xor (b16, *(&hannah));	/* replace with xor for sse */
  return 0;
}