With the advent of dynamic_function_ref the actual callee of such a ref
my vary. Optimizations should not assume to know the content of a
function referenced by dynamic_function_ref. Introduce
getReferencedFunctionOrNull which will return null for such function
refs. And getInitialReferencedFunction to return the referenced
function.
Use as appropriate.
rdar://50959798
Fixes the following warning:
```console
third_party/unsupported_toolchains/swift/src/swift/lib/SILOptimizer/Mandatory/OSLogOptimization.cpp:396:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
```
