cmd/compile: ensure FuncForPC works on closures that start with NOPs

cmd/compile: ensure FuncForPC works on closures that start with NOPs

A 0-sized no-op shouldn't prevent us from detecting that the first
instruction is from an inlined callee.

Update #58300

Change-Id: Ic5f6ed108c54a32c05e9b2264b516f2cc17e4619
Reviewed-on: https://go-review.googlesource.com/c/go/+/467977
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>