cmd/compile: function inlining produces incorrect results in certain conditions [1.12 backport]

[gopherbot]

@mvdan requested issue #30566 to be considered for backport to the next 1.12 minor release.

@gopherbot please backport to 1.12.

[gopherbot]

Change https://golang.org/cl/165858 mentions this issue: [release-branch.go1.12] cmd/compile: fix ordering for short-circuiting ops

[julieqiu]

@mvdan - there isn't a reason provided in the gopherbot message. Would you mind providing one for this backport?

[randall77]

There's an underlying bug which is fixed with the CL linked above. That bug has been in existence since at least 1.2.
However, with temporary variable reuse introduced in 1.12, it now causes actual incorrect results more often. So I think it qualifies for a backport.

[gopherbot]

Closed by merging f062f48 to release-branch.go1.12.