cmd/compile: don't compile some unexported, inlined functions

If a function is:

• inlined at all call sites
• not exported
• not referred to by value (any uses of the function are as a static call)
• not present in any interface types in its containing package

then we don't need to compile that function.

I suspect that these conditions are frequently met, cheap to check, and reasonably easy to implement.

I thought we had an issue for this, but I don't see one. I don't plan to work on this. I'm filing it in case someone else wants to. :)