cmd/compile/internal/inline: build call site table

cmd/compile/internal/inline: build call site table

Build up a table of (potentially) inlinable call sites during inline
heuristic analysis, and introduce a framework for analyzing each call
site to collect applicable flags (for example, is call nested in
loop). This patch doesn't include any of the flag analysis, just the
machinery to collect the callsites and a regression test harness.

Updates #61502.

Change-Id: Ieaf4a008ac9868e9762c63f5b59bd264dc71ab30
Reviewed-on: https://go-review.googlesource.com/c/go/+/511564
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>