[attrs] Simplify the convergent removal to directly use the pre-built

node set rather than walking the SCC directly.

This directly exposes the functions and has already had null entries
filtered out. We also don't need need to handle optnone as it has
already been handled in the caller -- we never try to remove convergent
when there are optnone functions in the SCC.

With this change, the code for removing convergent should work with the
new pass manager and a different SCC analysis.

llvm-svn: 260668