[InstCombine] Fixing wrong select folding in vectors with undef elements (#102244)

This PR fixes #98435.
`SimplifyDemandedVectorElts` mishandles the undef by assuming that
!isNullValue() means the condition is true.

By preventing any value that we're not certain equals 1 or 0, it avoids
having to make any particular choice by not demanding bits from a
particular branch with potentially picking a wrong value.

Proof: https://alive2.llvm.org/ce/z/r8CmEu

Author: jf-botto

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -1735,17 +1735,12 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
     APInt DemandedLHS(DemandedElts), DemandedRHS(DemandedElts);
     if (auto *CV = dyn_cast<ConstantVector>(Sel->getCondition())) {
       for (unsigned i = 0; i < VWidth; i++) {
-        // isNullValue() always returns false when called on a ConstantExpr.
-        // Skip constant expressions to avoid propagating incorrect information.
         Constant *CElt = CV->getAggregateElement(i);
-        if (isa<ConstantExpr>(CElt))
-          continue;
-        // TODO: If a select condition element is undef, we can demand from
-        // either side. If one side is known undef, choosing that side would
-        // propagate undef.
+
+        // isNullValue() always returns false when called on a ConstantExpr.
         if (CElt->isNullValue())
           DemandedLHS.clearBit(i);
-        else
+        else if (CElt->isOneValue())
           DemandedRHS.clearBit(i);
       }
     }

llvm/test/Transforms/InstCombine/pr98435.ll

@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=instcombine < %s 2>&1 | FileCheck %s
+
+define <2 x i1> @pr98435(<2 x i1> %val) {
+; CHECK-LABEL: define <2 x i1> @pr98435(
+; CHECK-SAME: <2 x i1> [[VAL:%.*]]) {
+; CHECK-NEXT:    [[VAL1:%.*]] = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> [[VAL]]
+; CHECK-NEXT:    ret <2 x i1> [[VAL1]]
+;
+  %val1 = select <2 x i1> <i1 undef, i1 true>, <2 x i1> <i1 true, i1 true>, <2 x i1> %val
+  ret <2 x i1> %val1
+}