cmd/compile/internal/pgo: use slices for in/out edges
cmd/compile/internal/pgo: use slices for in/out edges

Currently in the pprof Graph, a Node's in/out edges are
represented as maps, keyed by the source/destination Nodes. For a
Node it usually has a very small number of edges, so linear search
would be generally faster than map operations. Use slices and
linear search instead.

Change-Id: I5ab06be0c019373cd3a845b9f3282731372e9c4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/447802
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>
Run-TryBot: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go