RyuJIT - throughput improvements, PInvoke transitions tweaks: (#4842)

RyuJIT - throughput improvements, PInvoke transitions tweaks: (#4842)

* RyuJIT - throughput improvements, PInvoke transitions tweaks:

- Minor compilation throughput improvements:
     - Avoid redundant calls to getMethodAttribs in impMarkInlineCandidate by reusing the value from CALL_INFO
     - Avoid redundant calls to getFunctionEntryPoint in codegen by reusing the value computed during lower
     - Stop checking CORINFO_FLG_NATIVE flag for inliner observations. It is dead flag, not set anywhere
- Tweaks for helper-based PInvoke transitions (used CoreRT only today):
     - Stop emitting CORINFO_HELP_INIT_PINVOKE_FRAME. Just having the two simple begin/end helpers is better.
     - Stop passing size of arguments in secret register.
     - Enable direct calls for PInvoke targets

* Codereview feedback