[ConstProp] Don't fallthorugh for poison constants on vctp and active_lane_mask.

Given a poison constant as input, the dyn_cast to a ConstantInt would
fail so we would fall through to the generic code that attempts to fold
each element of the input vectors. The inputs to these intrinsics are
not vectors though, leading to a compile time crash. Instead bail out
properly for poison values by returning nullptr. This doesn't try to
define what poison means for these intrinsics.

Fixes #56945

Author: davemgreen

llvm/lib/Analysis/ConstantFolding.cpp

@@ -3120,7 +3120,7 @@ static Constant *ConstantFoldFixedVectorCall(
       }
       return ConstantVector::get(NCs);
     }
-    break;
+    return nullptr;
   }
   case Intrinsic::get_active_lane_mask: {
     auto *Op0 = dyn_cast<ConstantInt>(Operands[0]);
@@ -3139,7 +3139,7 @@ static Constant *ConstantFoldFixedVectorCall(
       }
       return ConstantVector::get(NCs);
     }
-    break;
+    return nullptr;
   }
   default:
     break;

llvm/test/Transforms/InstSimplify/ConstProp/ARM/mve-vctp.ll

@@ -259,7 +259,14 @@ entry:
   ret <2 x i1> %int
 }
 
-
+define <4 x float> @poisonc(<4 x float> %a) {
+entry:
+  %new0 = shl i1 0, 1
+  %last = zext i1 %new0 to i32
+  %var27 = call <4 x i1> @llvm.arm.mve.vctp32(i32 %last)
+  %var33 = select <4 x i1> %var27, <4 x float> %a, <4 x float> zeroinitializer
+  ret <4 x float> %var33
+}
 
 declare <2 x i1> @llvm.arm.mve.vctp64(i32)
 declare <4 x i1> @llvm.arm.mve.vctp32(i32)

llvm/test/Transforms/InstSimplify/ConstProp/active-lane-mask.ll

@@ -292,8 +292,14 @@ entry:
 }
 
 
-
-
+define <4 x float> @poisonc(<4 x float> %a, i32 %n) {
+entry:
+  %new0 = shl i1 0, 1
+  %last = zext i1 %new0 to i32
+  %var27 = call <4 x i1> @llvm.get.active.lane.mask.v4i1.i32(i32 %last, i32 1024)
+  %var33 = select <4 x i1> %var27, <4 x float> %a, <4 x float> zeroinitializer
+  ret <4 x float> %var33
+}
 
 declare <4 x i1> @llvm.get.active.lane.mask.v4i1.i32(i32, i32)
 declare <8 x i1> @llvm.get.active.lane.mask.v8i1.i32(i32, i32)