[AArch64] Add test for incorrect stxp register allocation (NFC)

This is a variant of #106380
without inline assembly.

Author: nikic

llvm/test/CodeGen/AArch64/arm64-ldxr-stxr.ll

@@ -353,6 +353,18 @@ define dso_local i32 @test_store_release_i64(i32, i64 %val, ptr %addr) {
   ret i32 %res
 }
 
+; The stxp result cannot be allocated to the same register as the inputs.
+; FIXME: This is a miscompile.
+define dso_local i32 @test_stxp_undef(ptr %p, i64 %x) nounwind {
+; CHECK-LABEL: test_stxp_undef:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    stxp w8, x8, x1, [x0]
+; CHECK-NEXT:    mov w0, w8
+; CHECK-NEXT:    ret
+  %res = call i32 @llvm.aarch64.stxp(i64 undef, i64 %x, ptr %p)
+  ret i32 %res
+}
+
 declare i32 @llvm.aarch64.stlxr.p0(i64, ptr) nounwind
 ;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
 ; FALLBACK: {{.*}}