[InstSimplify] sgt on i1s also encodes implication

Follow on to http://reviews.llvm.org/D13074, implementing something pointed out by Sanjoy. His truth table from his comment on that bug summarizes things well:
LHS | RHS | LHS >=s RHS | LHS implies RHS
0 | 0 | 1 (0 >= 0) | 1
0 | 1 | 1 (0 >= -1) | 1
1 | 0 | 0 (-1 >= 0) | 0
1 | 1 | 1 (-1 >= -1) | 1

The key point is that an "i1 1" is the value "-1", not "1".

Differential Revision: http://reviews.llvm.org/D13756

llvm-svn: 251597