cmd/compile/internal/inlheur: regionalize call site analysis
Refactor the code that looks for callsites to work on an arbitrary
region of IR nodes, as opposed to working on a function. No change in
semantics, this is just a refactoring in preparation for a later
change.
Change-Id: I73a61345c225dea566ffa6fa50f44dbaf9f1f32b
Reviewed-on: https://go-review.googlesource.com/c/go/+/530578 Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>