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

[Sanjay Patel]

Ping 2.

This is a miscompile that apparently occurs in the wild. 

Easy review - the fix doesn't add any lines of code other than comments. :)

http://reviews.llvm.org/D4909