Make it possible to disable method body rewriting

Linker should respect the NoOptimization MethodImpl flag and it should prevent it from messing with instructions in method bodies. Would make it easier to work around issues like https://github.com/mono/linker/issues/2181 or https://github.com/dotnet/runtime/issues/57010 that now require more creative workarounds.

I'm putting this in 6.0 because the method body rewriting is a relatively new feature, we're still finding bugs in it, and we'll likely find more when 6.0 ships.