[ValueTracking] Fix the identity case (LHS => RHS) when the LHS is false.

[ValueTracking] Fix the identity case (LHS => RHS) when the LHS is false.

Prior to this commit both of the added test cases were passing.  However, in the
latter case (test7) we were doing a lot more work to arrive at the same answer
(i.e., we were using isImpliedCondMatchingOperands() to determine the
implication.).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307400 91177308-0d34-0410-b5e6-96231b3b80d8