[InstCombine] Disable select known bits fold for vectors

This is not safe if the simplification ends up looking through
lane-crossing operations. For now, we don't have a good way to
limit this in computeKnownBits(), so just disable vector handling
entirely.

Fixes #97475.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

@@ -4049,7 +4049,9 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (CondVal->getType() == SI.getType() && isKnownInversion(FalseVal, TrueVal))
     return BinaryOperator::CreateXor(CondVal, FalseVal);
 
-  if (SelType->isIntOrIntVectorTy() &&
+  // For vectors, this transform is only safe if the simplification does not
+  // look through any lane-crossing operations. For now, limit to scalars only.
+  if (SelType->isIntegerTy() &&
       (!isa<Constant>(TrueVal) || !isa<Constant>(FalseVal))) {
     // Try to simplify select arms based on KnownBits implied by the condition.
     CondContext CC(CondVal);

llvm/test/Transforms/InstCombine/select-binop-cmp.ll

@@ -571,7 +571,10 @@ define <2 x i8> @select_xor_icmp_vec_bad(<2 x i8> %x, <2 x i8> %y, <2 x i8> %z)
 
 define <2 x i32> @vec_select_no_equivalence(<2 x i32> %x) {
 ; CHECK-LABEL: @vec_select_no_equivalence(
-; CHECK-NEXT:    ret <2 x i32> [[X:%.*]]
+; CHECK-NEXT:    [[X10:%.*]] = shufflevector <2 x i32> [[X:%.*]], <2 x i32> poison, <2 x i32> <i32 1, i32 0>
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq <2 x i32> [[X]], zeroinitializer
+; CHECK-NEXT:    [[S:%.*]] = select <2 x i1> [[COND]], <2 x i32> [[X10]], <2 x i32> [[X]]
+; CHECK-NEXT:    ret <2 x i32> [[S]]
 ;
   %x10 = shufflevector <2 x i32> %x, <2 x i32> undef, <2 x i32> <i32 1, i32 0>
   %cond = icmp eq <2 x i32> %x, zeroinitializer