Changes to processing of compiler generated methods lead to a state where we don't call constant prop and branch removal in all cases before we mark instructions of the method. This can lead to overmarking

This change fixes this by making sure that the branch removal executes on the method in all cases before we mark instructions of the method.

The change guarantees that all accesses to Body are after the constant prop/branch removal happened on the method.

This does have one possibly negative impact: the issue described in dotnet#2937 is now consistent and happens always.

Added tests.

Note that there's still a whole in analysis of compiler generated code around state machines, see dotnet#3087

Basically if there's a local function which is going to be removed due to branch removal and if the body of that method contains code which produces a warning due to generic parameter validation, such warning will always be generated even though it's "dead" code and even if it's suppressed via RUC or similar.

In such case the analysis can't figure out to which method the local function belongs (since the call site has been removed).