[IPO] Simplify the module inliner loop (NFC)

In the bottom-up inliner, we have a two-level nested "while" loop,
with the inner one grouping call sites with the same caller.  We need
to do so to keep CGSCC up to date.

Now, with the module inliner, we don't have any per-caller work.  We
don't update CGSCC.  Plus, the caller will likely keep changing as we
pop call sites in some priority order.

This patch simply removes the inner "while" loop while indenting its
body.  Further cleanup is possible, but that's left for follow-up
patches.

Differential Revision: https://reviews.llvm.org/D133969