cmd/compile/internal/pgo: Detect sample value position instead of hard-coding [1.20 backport]

[gopherbot]

@prattmic requested issue #58292 to be considered for backport to the next 1.20 minor release.

@gopherbot please backport to 1.20. Profiles with only a single sample type can an ICE.

[gopherbot]

Change https://go.dev/cl/466438 mentions this issue: [release-branch.go1.20] cmd/compile/internal/pgo: fix hard-coded PGO sample data position

[gopherbot]

Change https://go.dev/cl/467375 mentions this issue: [release-branch.go1.20] cmd/compile/internal/pgo: fix hard-coded PGO sample data position

[gopherbot]

Closed by merging 3a04b6e to release-branch.go1.20.