Patch over a really horrible bug in our vector builtins that showed up

recently when we started using direct conversion to model sign
extension. The __v16qi type we use for SSE v16i8 vectors is defined in
terms of 'char' which may or may not be signed! This causes us to
generate pmovsx and pmovzx depending on the setting of -funsigned-char.

This patch just forms an explicitly signed type and uses that to
formulate the sign extension. While this gets the correct behavior
(which we now verify with the enhanced test) this is just the tip of the
ice berg. Now that I know what to look for, I have found errors of this
sort *throughout* our vector code. Fortunately, this is the only
specific place where I know of users actively having their code
miscompiled by Clang due to this, so I'm keeping the fix for those users
minimal and targeted.

I'll be sending a proper email for discussion of how to fix these
systematically, what the implications are, and just how widely broken
this is... From what I can tell, we have never shipped a correct set of
builtin headers for x86 when users rely on -funsigned-char. Oops.

llvm-svn: 248980