[Analysis] bitreverse(undef) returns undef

[modocache]

Summary

The reverse of an artbitrary bitpattern is also an arbitrary
bitpattern.

[majnemer]

LGTM

[modocache]

Great, thanks for the review, @majnemer!

lib/Analysis/ConstantFolding.cpp

if (isa<ConstantVector>(Operands[0]) ||
         if (ConstantFP *FPOp =
                 dyn_cast_or_null<ConstantFP>(Op->getAggregateElement(0U)))
           return ConstantFoldSSEConvertToInt(FPOp->getValueAPF(),
                                              /*roundTowardZero=*/true, Ty);
       }
     }
 
     if (isa<UndefValue>(Operands[0])) {
-      if (IntrinsicID == Intrinsic::bswap)
+      if (IntrinsicID == Intrinsic::bswap ||
+          IntrinsicID == Intrinsic::bitreverse)
         return Operands[0];
       return nullptr;
     }
 
     return nullptr;
   }
 
   if (Operands.size() == 2) {

test/Transforms/InstCombine/bitreverse-fold.ll

 
 ; CHECK-LABEL: @reverse_neg1_i32(
 ; CHECK-NEXT: ret i32 -1
 define i32 @reverse_neg1_i32() {
   %x = call i32 @llvm.bitreverse.i32(i32 -1)
   ret i32 %x
 }
 
+; CHECK-LABEL: @reverse_undef_i32(
+; CHECK-NEXT: ret i32 undef
+define i32 @reverse_undef_i32() {
+  %x = call i32 @llvm.bitreverse.i32(i32 undef)
+  ret i32 %x
+}
+
 ; CHECK-LABEL: @reverse_false_i1(
 ; CHECK-NEXT: ret i1 false
 define i1 @reverse_false_i1() {
   %x = call i1 @llvm.bitreverse.i1(i1 false)
   ret i1 %x
 }
 
 ; CHECK-LABEL: @reverse_true_i1(
 ; CHECK-NEXT: ret i1 true
 define i1 @reverse_true_i1() {
   %x = call i1 @llvm.bitreverse.i1(i1 true)
   ret i1 %x
 }
 
+; CHECK-LABEL: @reverse_undef_i1(
+; CHECK-NEXT: ret i1 undef
+define i1 @reverse_undef_i1() {
+  %x = call i1 @llvm.bitreverse.i1(i1 undef)
+  ret i1 %x
+}
+
 ; CHECK-LABEL: @reverse_false_v2i1(
 ; CHECK-NEXT: ret <2 x i1> zeroinitializer
 define <2 x i1> @reverse_false_v2i1() {
   %x = call <2  x i1> @llvm.bitreverse.v2i1(<2 x i1> zeroinitializer)
   ret <2 x i1> %x
 }
 
 ; CHECK-LABEL: @reverse_true_v2i1(