Fix the SSE4 byte sign extension in a cleaner way, and more thoroughly

test that our intrinsics behave the same under -fsigned-char and
-funsigned-char.

This further testing uncovered that AVX-2 has a broken cmpgt for 8-bit
elements, and has for a long time. This is fixed in the same way as
SSE4 handles the case.

The other ISA extensions currently work correctly because they use
specific instruction intrinsics. As soon as they are rewritten in terms
of generic IR, they will need to add these special casts. I've added the
necessary testing to catch this however, so we shouldn't have to chase
it down again.

I considered changing the core typedef to be signed, but that seems like
a bad idea. Notably, it would be an ABI break if anyone is reaching into
the innards of the intrinsic headers and passing __v16qi on an API
boundary. I can't be completely confident that this wouldn't happen due
to a macro expanding in a lambda, etc., so it seems much better to leave
it alone. It also matches GCC's behavior exactly.

A fun side note is that for both GCC and Clang, -funsigned-char really
does change the semantics of __v16qi. To observe this, consider:

  % cat x.cc
  #include <smmintrin.h>
  #include <iostream>

  int main() {
    __v16qi a = { 1, -1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    __v16qi b = _mm_set1_epi8(-1);
    std::cout << (int)(a / b)[0] << ", " << (int)(a / b)[1] << '\n';
  }
  % clang++ -o x x.cc && ./x
  -1, 1
  % clang++ -funsigned-char -o x x.cc && ./x
  0, 1

However, while this may be surprising, both Clang and GCC agree.

Differential Revision: http://reviews.llvm.org/D13324

llvm-svn: 249097