Fix assert in NativeAOT when handling data flow on compiler generated code. (#73085)

Fix assert in NativeAOT when handling data flow on compiler generated code. (#73085)

The problem occurs when an entire type/assembly is preserved through explicit rooting (command line, rd.xml, ...). If such type contains a local function (for example) which is only called from a branch which is going to be removed by constant-prop/branch removal the internal tracking of compiler generated methods will see this local function as orphaned (not belonging to any user method). This leads to a case where we will try to run data flow on the local function - but that should never happen for compiler generated methods directly -> assert.

The fix is (just like in the linker), to never run data flow on compiler generated methods directly - they should only run data flow as part of their respective user method.

Fixes https://github.com/dotnet/runtime/issues/73027