cmd/compile/internal/pgo: move pprof graph to internal package

graph.go is a simplified fork of github.com/google/pprof/internal/graph,
which is used as an intermediate data structure to construct the final
graph exported by package pgo (IRGraph).

Exporting both is a bit confusing as the former is unused outside of the
package. Since the naming is also similar, move graph.go to its own
package entirely.

Change-Id: I2bccb3ddb6c3f63afb869ea9cf34d2a261cad058
Reviewed-on: https://go-review.googlesource.com/c/go/+/494437
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>

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