Recommit "[VPlan] Mark Select VPInstructions as not having sideeffects."

With #70253 landed, selects for reduction results are explicitly used by
ComputeReductionResult and Selects can be marked as not having
side-effects again.

This reverts the revert commit 1730329.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -121,6 +121,7 @@ bool VPRecipeBase::mayHaveSideEffects() const {
   case VPInstructionSC:
     switch (cast<VPInstruction>(this)->getOpcode()) {
     case Instruction::ICmp:
+    case Instruction::Select:
     case VPInstruction::Not:
     case VPInstruction::CalculateTripCountMinusVF:
     case VPInstruction::CanonicalIVIncrementForPart:

llvm/test/Transforms/LoopVectorize/reduction-small-size.ll

@@ -11,15 +11,12 @@ define i8 @PR34687(i1 %c, i32 %x, i32 %n) {
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    [[N_MOD_VF:%.*]] = urem i32 [[N]], 4
 ; CHECK-NEXT:    [[N_VEC:%.*]] = sub i32 [[N]], [[N_MOD_VF]]
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i1> poison, i1 [[C:%.*]], i64 0
-; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i1> [[BROADCAST_SPLATINSERT]], <4 x i1> poison, <4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[BROADCAST_SPLATINSERT1:%.*]] = insertelement <4 x i32> poison, i32 [[X:%.*]], i64 0
 ; CHECK-NEXT:    [[BROADCAST_SPLAT2:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT1]], <4 x i32> poison, <4 x i32> zeroinitializer
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i32> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP4:%.*]], [[VECTOR_BODY]] ]
-; CHECK-NEXT:    [[TMP0:%.*]] = select <4 x i1> [[BROADCAST_SPLAT]], <4 x i32> undef, <4 x i32> <i32 1, i32 1, i32 1, i32 1>
 ; CHECK-NEXT:    [[TMP1:%.*]] = and <4 x i32> [[VEC_PHI]], <i32 255, i32 255, i32 255, i32 255>
 ; CHECK-NEXT:    [[TMP2:%.*]] = add <4 x i32> [[TMP1]], [[BROADCAST_SPLAT2]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = trunc <4 x i32> [[TMP2]] to <4 x i8>
@@ -40,7 +37,7 @@ define i8 @PR34687(i1 %c, i32 %x, i32 %n) {
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[I:%.*]] = phi i32 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[I_NEXT:%.*]], [[IF_END:%.*]] ]
 ; CHECK-NEXT:    [[R:%.*]] = phi i32 [ [[BC_MERGE_RDX]], [[SCALAR_PH]] ], [ [[R_NEXT:%.*]], [[IF_END]] ]
-; CHECK-NEXT:    br i1 [[C]], label [[IF_THEN:%.*]], label [[IF_END]]
+; CHECK-NEXT:    br i1 [[C:%.*]], label [[IF_THEN:%.*]], label [[IF_END]]
 ; CHECK:       if.then:
 ; CHECK-NEXT:    [[T0:%.*]] = sdiv i32 undef, undef
 ; CHECK-NEXT:    br label [[IF_END]]