[PATCH] fix a logic bug in x86 vector codegen: sext (zext (x) ) != sext (x) (PR20472)

[Nadav Rotem]

LGTM!

Sanjay, you can commit changes like this without waiting for pre-commit code review (See item #3 in http://llvm.org/docs/DeveloperPolicy.html#code-reviews).

Thanks,
Nadav

http://reviews.llvm.org/D4909