[InstSimplify] sle on i1 also encodes implication

We already support SGE, so the same logic should hold for SLE with
the LHS and RHS swapped.

I didn't see this in the wild. Just happened to walk past this code
and thought it was odd that it was asymmetric in what condition
codes it handled.

Reviewed By: spatel, reames

Differential Revision: https://reviews.llvm.org/D131805