[SimplifyCFG] Add missing implications to isImpliedTrueByMatchingCmp.

Summary: [u|s]gt and [u|s]lt imply [u|s]ge and [u|s]le are true, respectively.
I've simplified the existing tests and added additional tests to cover the new
cases mentioned above.  I've also added tests for all the cases where the
first compare doesn't imply anything about the second compare.

llvm-svn: 267171