Utilize VN-based non-null knowledge better

Previously, global non-null assertion propagation would give up
on any non-"ADD(LCL_VAR, CONST)"-like trees. This is correct for
actual assertion-based propagation (since we record assertions
based on conservative VNs and those propagate only through locals),
but is unnecessarily conservative when it comes to utilizing
non-nullness provided by VN.

Fix this by moving the IR checks after the VN check.

src/coreclr/jit/assertionprop.cpp

@@ -4297,23 +4297,11 @@ GenTree* Compiler::optAssertionProp_Ind(ASSERT_VALARG_TP assertions, GenTree* tr
         return nullptr;
     }
 
-    // Check for add of a constant.
-    GenTree* op1 = tree->AsIndir()->Addr();
-    if ((op1->gtOper == GT_ADD) && (op1->AsOp()->gtOp2->gtOper == GT_CNS_INT))
-    {
-        op1 = op1->AsOp()->gtOp1;
-    }
-
-    if (op1->gtOper != GT_LCL_VAR)
-    {
-        return nullptr;
-    }
-
 #ifdef DEBUG
     bool           vnBased = false;
     AssertionIndex index   = NO_ASSERTION_INDEX;
 #endif
-    if (optAssertionIsNonNull(op1, assertions DEBUGARG(&vnBased) DEBUGARG(&index)))
+    if (optAssertionIsNonNull(tree->AsIndir()->Addr(), assertions DEBUGARG(&vnBased) DEBUGARG(&index)))
     {
 #ifdef DEBUG
         if (verbose)
@@ -4359,19 +4347,28 @@ bool Compiler::optAssertionIsNonNull(GenTree*         op,
                                      ASSERT_VALARG_TP assertions DEBUGARG(bool* pVnBased)
                                          DEBUGARG(AssertionIndex* pIndex))
 {
+    if (op->OperIs(GT_ADD) && op->AsOp()->gtGetOp2()->IsCnsIntOrI() &&
+        !fgIsBigOffset(op->AsOp()->gtGetOp2()->AsIntCon()->IconValue()))
+    {
+        op = op->AsOp()->gtGetOp1();
+    }
+
     bool vnBased = (!optLocalAssertionProp && vnStore->IsKnownNonNull(op->gtVNPair.GetConservative()));
 #ifdef DEBUG
+    *pIndex   = NO_ASSERTION_INDEX;
     *pVnBased = vnBased;
 #endif
 
     if (vnBased)
     {
-#ifdef DEBUG
-        *pIndex = NO_ASSERTION_INDEX;
-#endif
         return true;
     }
 
+    if (!op->OperIs(GT_LCL_VAR))
+    {
+        return false;
+    }
+
     AssertionIndex index = optAssertionIsNonNullInternal(op, assertions DEBUGARG(pVnBased));
 #ifdef DEBUG
     *pIndex = index;
@@ -4494,16 +4491,13 @@ AssertionIndex Compiler::optAssertionIsNonNullInternal(GenTree*         op,
  */
 GenTree* Compiler::optNonNullAssertionProp_Call(ASSERT_VALARG_TP assertions, GenTreeCall* call)
 {
-    if ((call->gtFlags & GTF_CALL_NULLCHECK) == 0)
+    if (!call->NeedsNullCheck())
     {
         return nullptr;
     }
+
     GenTree* op1 = call->gtArgs.GetThisArg()->GetNode();
     noway_assert(op1 != nullptr);
-    if (op1->gtOper != GT_LCL_VAR)
-    {
-        return nullptr;
-    }
 
 #ifdef DEBUG
     bool           vnBased = false;
@@ -4524,6 +4518,7 @@ GenTree* Compiler::optNonNullAssertionProp_Call(ASSERT_VALARG_TP assertions, Gen
         noway_assert(call->gtFlags & GTF_SIDE_EFFECT);
         return call;
     }
+
     return nullptr;
 }