[InstSimplify] sle on i1 also encodes implication

[craig.topper]

Summary

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.

[spatel]

LGTM

llvm/lib/Analysis/InstructionSimplify.cpp

 //===- InstructionSimplify.cpp - Fold instruction operands ----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
case ICmpInst::ICMP_SGE:
     ///  1  |  1  |  1 (-1 >= -1) |  1
     if (isImpliedCondition(LHS, RHS, Q.DL).value_or(false))
       return getTrue(ITy);
     break;
   case ICmpInst::ICMP_ULE:
     if (isImpliedCondition(LHS, RHS, Q.DL).value_or(false))
       return getTrue(ITy);
     break;
+  case ICmpInst::ICMP_SLE:
+    /// SLE follows the same logic as SGE with the LHS and RHS swapped.
+    if (isImpliedCondition(RHS, LHS, Q.DL).value_or(false))
+      return getTrue(ITy);
+    break;
   }
 
   return nullptr;
 }
 
 /// Try hard to fold icmp with zero RHS because this is a common case.
 static Value *simplifyICmpWithZero(CmpInst::Predicate Pred, Value *LHS,
                                    Value *RHS, const SimplifyQuery &Q) {

llvm/test/Transforms/InstSimplify/implies.ll

 ; CHECK:         ret i1 true
 ;
   %iplus1 = add nsw nuw i32 %i, 1
   %var29 = icmp ult i32 %i, %length.i
   %var30 = icmp ult i32 %iplus1, %length.i
   %res = icmp sge i1 %var30, %var29
   ret i1 %res
 }
+
+; X <=(s) Y == Y ==> X (i1 1 becomes -1 for reasoning)
+define i1 @test_sle(i32 %length.i, i32 %i) {
+; CHECK-LABEL: @test_sle(
+; CHECK-NEXT:    ret i1 true
+;
+  %iplus1 = add nsw nuw i32 %i, 1
+  %var29 = icmp ult i32 %i, %length.i
+  %var30 = icmp ult i32 %iplus1, %length.i
+  %res = icmp sle i1 %var29, %var30
+  ret i1 %res
+}