Don't do opportunistic ISA lightup if there are plans to run the AltJ…

…it code (#110419)

* Don't do opportunistic ISA lightup if there are plans to run the AltJit code

* Ensure that we can compile in Release builds

src/coreclr/jit/compiler.cpp

@@ -6048,7 +6048,16 @@ int Compiler::compCompile(CORINFO_MODULE_HANDLE classPtr,
     // but it requires moving the whole set of logic down into compCompileHelper after compInitOptions has
     // run and we're going to end up exiting early if JIT_FLAG_ALT_JIT and opts.altJit don't match anyways
 
-    if (!info.compMatchedVM || compileFlags->IsSet(JitFlags::JIT_FLAG_ALT_JIT))
+    bool enableAvailableIsas = !info.compMatchedVM;
+
+#ifdef DEBUG
+    if (compileFlags->IsSet(JitFlags::JIT_FLAG_ALT_JIT) && JitConfig.RunAltJitCode() == 0)
+    {
+        enableAvailableIsas = true;
+    }
+#endif // DEBUG
+
+    if (enableAvailableIsas)
     {
         CORINFO_InstructionSetFlags instructionSetFlags;