cmd/compile/internal/pgo: match on call line offsets

Rather than matching calls to edges in the profile based directly on
line number in the source file, use the line offset from the start of
the function. This makes matching robust to changes in the source file
above the function containing the call.

The start line in the profile comes from Function.start_line, which is
included in Go pprof output since CL 438255.

Currently it is an error if no samples set start_line to help users
detect profiles missing this information. In the future, we should
fallback to using absolute lines, which is better than nothing.

For #55022.

Change-Id: Ie621950cfee1fef8fb200907a2a3f1ded41d04fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/447315
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>

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