[PhaseOrdering] Add test for memset DSE (NFC)

This checks for the interaction with #145474.

Author: nikic

llvm/test/Transforms/PhaseOrdering/memset-dse.ll

@@ -0,0 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -O2 < %s | FileCheck %s
+
+declare void @do_something(ptr %a)
+
+; We want only one memset in the if block (rather than only one memset in
+; the entry block).
+define void @test(i1 %c) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: i1 [[C:%.*]]) local_unnamed_addr {
+; CHECK-NEXT:    [[A:%.*]] = alloca [32 x i8], align 1
+; CHECK-NEXT:    br i1 [[C]], label %[[IF:.*]], label %[[COMMON_RET:.*]]
+; CHECK:       [[COMMON_RET]]:
+; CHECK-NEXT:    ret void
+; CHECK:       [[IF]]:
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(32) [[A]], i8 0, i64 32, i1 false)
+; CHECK-NEXT:    call void @do_something(ptr nonnull [[A]])
+; CHECK-NEXT:    br label %[[COMMON_RET]]
+;
+  %a = alloca [32 x i8]
+  call void @llvm.memset(ptr %a, i8 0, i64 32, i1 false)
+  br i1 %c, label %if, label %else
+if:
+  call void @llvm.memset(ptr %a, i8 0, i64 32, i1 false)
+  call void @do_something(ptr %a)
+  ret void
+
+else:
+  ret void
+}