Enable (sext x) == C --> x == (trunc C) combine

Extend the existing code which handles this for zext. This makes this
more useful for targets with ZeroOrNegativeOne BooleanContent and
obsoletes a custom combine SI uses for i1 setcc (sext(i1), 0, setne)
since the constant will now be shrunk to i1.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224691 91177308-0d34-0410-b5e6-96231b3b80d8