[DSE] Make test more robust (NFC)

If the allocation is not captured, then all the stores before the
ret are dead anyway.

Author: nikic

llvm/test/Transforms/DeadStoreElimination/memset-unknown-sizes.ll

@@ -6,37 +6,37 @@ declare i8* @_Znwm() local_unnamed_addr #0
 ; Function Attrs: argmemonly nounwind willreturn writeonly
 declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i1 immarg) #1
 
-define void @test1(i1 %c, i64 %N) {
+define i8* @test1(i1 %c, i64 %N) {
 ; CHECK-LABEL: @test1(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[COND_TRUE_I_I_I:%.*]], label [[COND_END_I_I_I:%.*]]
 ; CHECK:       cond.true.i.i.i:
-; CHECK-NEXT:    ret void
+; CHECK-NEXT:    ret i8* null
 ; CHECK:       cond.end.i.i.i:
 ; CHECK-NEXT:    [[ALLOC:%.*]] = tail call noalias nonnull i8* @_Znam() #[[ATTR2:[0-9]+]]
 ; CHECK-NEXT:    [[ALLOC_BC:%.*]] = bitcast i8* [[ALLOC]] to i64*
 ; CHECK-NEXT:    tail call void @llvm.memset.p0i8.i64(i8* nonnull align 8 [[ALLOC]], i8 0, i64 [[N:%.*]], i1 false) #[[ATTR3:[0-9]+]]
 ; CHECK-NEXT:    store i64 0, i64* [[ALLOC_BC]], align 8
-; CHECK-NEXT:    ret void
+; CHECK-NEXT:    ret i8* [[ALLOC]]
 ;
 entry:
   br i1 %c, label %cond.true.i.i.i, label %cond.end.i.i.i
 
 cond.true.i.i.i:                                  ; preds = %entry
-  ret void
+  ret i8* null
 
 cond.end.i.i.i:                                   ; preds = %entry
   %alloc = tail call noalias nonnull i8* @_Znam() #2
   %alloc.bc = bitcast i8* %alloc to i64*
   tail call void @llvm.memset.p0i8.i64(i8* nonnull align 8 %alloc, i8 0, i64 %N, i1 false) #3
   store i64 0, i64* %alloc.bc, align 8
-  ret void
+  ret i8* %alloc
 }
 
 declare i8* @_Znam()
 
 
-define void @test2(i1 %c, i64 %N) {
+define i8* @test2(i1 %c, i64 %N) {
 ; CHECK-LABEL: @test2(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[C:%.*]], label [[CLEANUP_CONT104:%.*]], label [[IF_THEN:%.*]]
@@ -46,9 +46,9 @@ define void @test2(i1 %c, i64 %N) {
 ; CHECK-NEXT:    [[ALLOC_BC:%.*]] = bitcast i8* [[ALLOC]] to i64*
 ; CHECK-NEXT:    store i64 0, i64* [[ALLOC_BC]], align 8
 ; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* nonnull align 8 [[ALLOC]], i8 0, i64 [[MUL]], i1 false) #[[ATTR3]]
-; CHECK-NEXT:    ret void
+; CHECK-NEXT:    ret i8* [[ALLOC]]
 ; CHECK:       cleanup.cont104:
-; CHECK-NEXT:    ret void
+; CHECK-NEXT:    ret i8* null
 ;
 entry:
   br i1 %c, label %cleanup.cont104, label %if.then
@@ -59,10 +59,10 @@ if.then:                                          ; preds = %entry
   %alloc.bc = bitcast i8* %alloc to i64*
   store i64 0, i64* %alloc.bc, align 8
   call void @llvm.memset.p0i8.i64(i8* nonnull align 8 %alloc, i8 0, i64 %mul, i1 false) #3
-  ret void
+  ret i8* %alloc
 
 cleanup.cont104:                                  ; preds = %entry
-  ret void
+  ret i8* null
 }
 
 attributes #0 = { "use-soft-float"="false" }