Allow AltJit for matched VM to also use otherwise unsupported ISAs (d…

…otnet#110320)

src/coreclr/jit/compiler.cpp

@@ -6042,8 +6042,13 @@ int Compiler::compCompile(CORINFO_MODULE_HANDLE classPtr,
     // Note that it might be better to do this immediately when setting the JIT flags in CILJit::compileMethod()
     // (when JitFlags::SetFromFlags() is called), but this is close enough. (To move this logic to
     // CILJit::compileMethod() would require moving the info.compMatchedVM computation there as well.)
+    //
+    // We additionally want to do this for AltJit so that we can validate ISAs that the underlying CPU may
+    // not support directly. Doing this check later, after opts.altJit has been initialized might be better
+    // 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)
+    if (!info.compMatchedVM || compileFlags->IsSet(JitFlags::JIT_FLAG_ALT_JIT))
     {
         CORINFO_InstructionSetFlags instructionSetFlags;