[InstCombine] improve perf by not creating a known non-canonical instruction

Op1 (RHS) is a constant, so putting it on the LHS makes us churn through visitICmp
an extra time to canonicalize it:

INSTCOMBINE ITERATION #1 on cmpnot
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %notx = xor i8 %x, -1
IC: Visiting:   %cmp = icmp sgt i8 %notx, 42
IC: Old =   %cmp = icmp sgt i8 %notx, 42
    New =   <badref> = icmp sgt i8 -43, %x
IC: ADD:   %cmp = icmp sgt i8 -43, %x
IC: ERASE   %1 = icmp sgt i8 %notx, 42
IC: ADD:   %notx = xor i8 %x, -1
IC: DCE:   %notx = xor i8 %x, -1
IC: ERASE   %notx = xor i8 %x, -1
IC: Visiting:   %cmp = icmp sgt i8 -43, %x
IC: Mod =   %cmp = icmp sgt i8 -43, %x
    New =   %cmp = icmp slt i8 %x, -43
IC: ADD:   %cmp = icmp slt i8 %x, -43
IC: Visiting:   %cmp = icmp slt i8 %x, -43
IC: Visiting:   ret i1 %cmp

If we create the swapped ICmp directly, we go faster:

INSTCOMBINE ITERATION #1 on cmpnot
IC: ADDING: 3 instrs to worklist
IC: Visiting:   %notx = xor i8 %x, -1
IC: Visiting:   %cmp = icmp sgt i8 %notx, 42
IC: Old =   %cmp = icmp sgt i8 %notx, 42
    New =   <badref> = icmp slt i8 %x, -43
IC: ADD:   %cmp = icmp slt i8 %x, -43
IC: ERASE   %1 = icmp sgt i8 %notx, 42
IC: ADD:   %notx = xor i8 %x, -1
IC: DCE:   %notx = xor i8 %x, -1
IC: ERASE   %notx = xor i8 %x, -1
IC: Visiting:   %cmp = icmp slt i8 %x, -43
IC: Visiting:   ret i1 %cmp




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304558 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rotateright

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -4508,13 +4508,16 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
                           Builder->CreateAnd(A, B),
                           Op1);
 
-    // ~x < ~y --> y < x
-    // ~x < cst --> ~cst < x
+    // ~X < ~Y --> Y < X
+    // ~X < C -->  X > ~C
     if (match(Op0, m_Not(m_Value(A)))) {
       if (match(Op1, m_Not(m_Value(B))))
         return new ICmpInst(I.getPredicate(), B, A);
+
+      // FIXME: Use m_APInt to include splat vector constants.
       if (ConstantInt *RHSC = dyn_cast<ConstantInt>(Op1))
-        return new ICmpInst(I.getPredicate(), ConstantExpr::getNot(RHSC), A);
+        return new ICmpInst(I.getSwappedPredicate(), A,
+                            ConstantExpr::getNot(RHSC));
     }
 
     Instruction *AddI = nullptr;