[x86] Remove the old vector shuffle lowering code and its flag.

The new shuffle lowering has been the default for some time. I've
enabled the new legality testing by default with no really blocking
regressions. I've fuzz tested this very heavily (many millions of fuzz
test cases have passed at this point). And this cleans up a ton of code.
=]

Thanks again to the many folks that helped with this transition. There
was a lot of work by others that went into the new shuffle lowering to
make it really excellent.

In case you aren't using a diff algorithm that can handle this:
  X86ISelLowering.cpp: 22 insertions(+), 2940 deletions(-)

llvm-svn: 229964