cmd/compile/internal/inline: refactor fixpoint algorithm

cmd/compile/internal/inline: refactor fixpoint algorithm

[Re-land of CL 567695 without further changes.]

This CL refactors the interleaved fixpoint algorithm so that calls can
be inlined in any order. This has no immediate effect, but it will
allow a subsequent CL to prioritize calls by inlheur score.

Change-Id: I8fd6748f0347fd696daee815bf7c9c183572c1ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/573096
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>