cmd/compile: incorrect loop trip count [1.26 backport]

[gopherbot]

@Jorropo requested issue #78303 to be considered for backport to the next 1.26 minor release.

@gopherbot please open backport issues for 1.25 and 1.26

This is a misscompilation that yields different runtime behavior than the go spec requires.

[gopherbot]

Change https://go.dev/cl/760041 mentions this issue: cmd/compile: do not invert loops that would overflow or underflow

[gopherbot]

Change https://go.dev/cl/760040 mentions this issue: cmd/compile: cleanup downward counting loop code

[gopherbot]

Change https://go.dev/cl/766982 mentions this issue: [release-branch.go1.26] fix incorrect loop trip counts