[NFC] Pre-calculate SCC IR counts in size remarks.

Same vein as the previous commits. Pre-calculate the size of
the module and use that to decide if we're going to emit a
remark.

This one comes with a FIXME and TODO. First off, CallGraphSCC
and CallGraphNode don't have a getInstructionCount function. So,
for now, we do the same thing as in a module pass.

Second off, we're not really saving anything here yet, because
as before, I need to change emitInstrCountChangedRemark to take
in a delta. Keeping the patches small though, so that's coming up
next.

5/6

llvm-svn: 341249