[FunctionAttrs][NPM] Fix handling of convergent

The legacy pass didn't properly detect indirect calls.

We can still remove the convergent attribute when there are indirect
calls. The LangRef says:

> When it appears on a call/invoke, the convergent attribute indicates
that we should treat the call as though we’re calling a convergent
function. This is particularly useful on indirect calls; without this we
may treat such calls as though the target is non-convergent.

So don't skip handling of convergent when there are unknown calls.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D89826