cmd/compile: go1.22+ cmd with go.mod 1.21 generates per-loop variable when using line directive [1.25 backport]

[gopherbot]

@xieyuschen requested issue #77248 to be considered for backport to the next 1.25 minor release.

@gopherbot please open backport issues.

The issue breaks the compatible behavior mentioned in loopvar docs. It's a corner case but I believe the use case(see background) is valid

[gopherbot]

Change https://go.dev/cl/739020 mentions this issue: [release-branch.go1.25] cmd/compile: fix loopvar version detection with line directives