runtime/pprof: clarify recursive inline heuristic

Following CL 226818, the compiler will allow inlining a single cycle in
an inline chain. Immediately-recursive functions are still disallowed,
which is what this heuristic refers to.

Add a regression test for this case.

Note that in addition to this check, if the compiler were to inline
multiple cycles via a loop (i.e., rather than appending duplicate code),
much more work would be required here to handle a single address
appearing in multiple different inline frames.

Updates #29737

Change-Id: I88de15cfbeabb9c04381e1c12cc36778623132a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/227346


Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>