Optimize sext/zext insertion algorithm in back-end.

With this optimization, we will not always insert zext for values crossing
basic blocks, but insert sext if the users of a value crossing basic block
has preference of sign predicate.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218101 91177308-0d34-0410-b5e6-96231b3b80d8