ART: Avoid some tidy nullptr warnings

Sprinkle some (D)CHECKs to tell tidy our expectations.

Bug: 32619234
Test: m test-art-host
Change-Id: I315b1602b20475402dd8383e1accc49e5a63eb5c
diff --git a/compiler/optimizing/code_generator.cc b/compiler/optimizing/code_generator.cc
index d8e442c..c2e83cd 100644
--- a/compiler/optimizing/code_generator.cc
+++ b/compiler/optimizing/code_generator.cc
@@ -1126,6 +1126,7 @@
   if (osr) {
     DCHECK_EQ(info->GetSuspendCheck(), instruction);
     DCHECK(info->IsIrreducible());
+    DCHECK(environment != nullptr);
     if (kIsDebugBuild) {
       for (size_t i = 0, environment_size = environment->Size(); i < environment_size; ++i) {
         HInstruction* in_environment = environment->GetInstructionAt(i);