[LoongArch] Precommit test case to show bug in LoongArchISelDagToDag

The optimization level should not be restored into O2.

Author: SixWeining

llvm/test/CodeGen/LoongArch/isel-optnone.ll

@@ -0,0 +1,13 @@
+; REQUIRES: asserts
+; RUN: llc %s -O0 -mtriple=loongarch64 -o /dev/null -debug-only=isel 2>&1 | FileCheck %s
+
+define void @fooOptnone() #0 {
+; CHECK: Changing optimization level for Function fooOptnone
+; CHECK: Before: -O2 ; After: -O0
+
+; CHECK: Restoring optimization level for Function fooOptnone
+; CHECK: Before: -O0 ; After: -O2
+  ret void
+}
+
+attributes #0 = { nounwind optnone noinline }