[InstCombine] Consolidate another fold into select value equivalence

We had a separate fold that handled just the trivial case where
we're replacing exactly the argument of the select. Handle this
in select value equivalence by relaxing the infinite loop protection
to allow a replacement of a non-constant with a constant.

This also fixes #113301,
as the separate fold did not handle undef values correctly.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -1348,7 +1348,7 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
     // If we will be able to evaluate f(Y) to a constant, we can allow undef,
     // otherwise Y cannot be undef as we might pick different values for undef
     // in the cmp and in f(Y).
-    if (TrueVal == OldOp)
+    if (TrueVal == OldOp && (isa<Constant>(OldOp) || !isa<Constant>(NewOp)))
       return nullptr;
 
     if (Value *V = simplifyWithOpReplaced(TrueVal, OldOp, NewOp, SQ,
@@ -1925,17 +1925,6 @@ Instruction *InstCombinerImpl::foldSelectInstWithICmp(SelectInst &SI,
   ICmpInst::Predicate Pred = ICI->getPredicate();
   Value *CmpLHS = ICI->getOperand(0);
   Value *CmpRHS = ICI->getOperand(1);
-  if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS) && !isa<Constant>(CmpLHS)) {
-    if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
-      // Transform (X == C) ? X : Y -> (X == C) ? C : Y
-      replaceOperand(SI, 1, CmpRHS);
-      Changed = true;
-    } else if (CmpLHS == FalseVal && Pred == ICmpInst::ICMP_NE) {
-      // Transform (X != C) ? Y : X -> (X != C) ? Y : C
-      replaceOperand(SI, 2, CmpRHS);
-      Changed = true;
-    }
-  }
 
   if (Instruction *NewSel = foldSelectICmpEq(SI, ICI, *this))
     return NewSel;

llvm/test/Transforms/InstCombine/select-value-equivalence.ll

@@ -322,12 +322,11 @@ define <2 x i8> @select_vec_op_const_no_undef(<2 x i8> %x) {
   ret <2 x i8> %xr
 }
 
-; FIXME: This is a miscompile.
 define <2 x i8> @select_vec_op_const_undef(<2 x i8> %x) {
 ; CHECK-LABEL: define <2 x i8> @select_vec_op_const_undef(
 ; CHECK-SAME: <2 x i8> [[X:%.*]]) {
 ; CHECK-NEXT:    [[XZ:%.*]] = icmp eq <2 x i8> [[X]], <i8 1, i8 undef>
-; CHECK-NEXT:    [[XR:%.*]] = select <2 x i1> [[XZ]], <2 x i8> <i8 1, i8 undef>, <2 x i8> <i8 4, i8 3>
+; CHECK-NEXT:    [[XR:%.*]] = select <2 x i1> [[XZ]], <2 x i8> [[X]], <2 x i8> <i8 4, i8 3>
 ; CHECK-NEXT:    ret <2 x i8> [[XR]]
 ;
   %xz = icmp eq <2 x i8> %x, <i8 1, i8 undef>