[DSE] Add test for interaction with return-only captures (NFC)

Regression test for the miscompile reported at:
#125880 (comment)

Author: nikic

llvm/test/Transforms/DeadStoreElimination/captures-ret-only.ll

@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=dse < %s | FileCheck %s
+
+declare ptr @passthrough(ptr)
+
+define i16 @ret_only() {
+; CHECK-LABEL: define i16 @ret_only() {
+; CHECK-NEXT:    [[A:%.*]] = alloca i16, align 1
+; CHECK-NEXT:    store i16 1, ptr [[A]], align 1
+; CHECK-NEXT:    [[CALL:%.*]] = call ptr @passthrough(ptr readnone captures(ret: address, provenance) [[A]])
+; CHECK-NEXT:    [[V:%.*]] = load i16, ptr [[CALL]], align 1
+; CHECK-NEXT:    ret i16 [[V]]
+;
+  %a = alloca i16, align 1
+  store i16 1, ptr %a, align 1
+  %call = call ptr @passthrough(ptr readnone captures(ret: address, provenance) %a)
+  %v = load i16, ptr %call, align 1
+  ret i16 %v
+}
+
+define i16 @ret_has_more_components() {
+; CHECK-LABEL: define i16 @ret_has_more_components() {
+; CHECK-NEXT:    [[A:%.*]] = alloca i16, align 1
+; CHECK-NEXT:    store i16 1, ptr [[A]], align 1
+; CHECK-NEXT:    [[CALL:%.*]] = call ptr @passthrough(ptr readnone captures(address, ret: address, provenance) [[A]])
+; CHECK-NEXT:    [[V:%.*]] = load i16, ptr [[CALL]], align 1
+; CHECK-NEXT:    ret i16 [[V]]
+;
+  %a = alloca i16, align 1
+  store i16 1, ptr %a, align 1
+  %call = call ptr @passthrough(ptr readnone captures(address, ret: address, provenance) %a)
+  %v = load i16, ptr %call, align 1
+  ret i16 %v
+}