JIT: Avoid creating impossible equality assertions (#118586)

Fix #113940

Author: jakobbotsch

src/coreclr/jit/assertionprop.cpp

@@ -1232,9 +1232,18 @@ AssertionIndex Compiler::optCreateAssertion(GenTree* op1, GenTree* op2, optAsser
                     if (op2->OperIs(GT_CNS_INT))
                     {
                         ssize_t iconVal = op2->AsIntCon()->IconValue();
-                        if (varTypeIsSmall(lclVar) && op1->OperIs(GT_STORE_LCL_VAR))
+                        if (varTypeIsSmall(lclVar))
                         {
-                            iconVal = optCastConstantSmall(iconVal, lclVar->TypeGet());
+                            ssize_t truncatedIconVal = optCastConstantSmall(iconVal, lclVar->TypeGet());
+                            if (!op1->OperIs(GT_STORE_LCL_VAR) && (truncatedIconVal != iconVal))
+                            {
+                                // This assertion would be saying that a small local is equal to a value
+                                // outside its range. It means this block is unreachable. Avoid creating
+                                // such impossible assertions which can hit assertions in other places.
+                                goto DONE_ASSERTION;
+                            }
+
+                            iconVal = truncatedIconVal;
                             if (!optLocalAssertionProp)
                             {
                                 assertion.op2.vn = vnStore->VNForIntCon(static_cast<int>(iconVal));