cmd/compile: `switch` statement on a custom `int32` type with negative values behaves differently in two consecutive calls [1.12 backport]

[gopherbot]

@josharian requested issue #32560 to be considered for backport to the next 1.12 minor release.

@gopherbot please open backport issues. This is an important compiler correctness bug with a safe, localized, minimal fix.

[gopherbot]

Change https://golang.org/cl/181978 mentions this issue: [release-branch.go1.12] cmd/compile: fix range analysis of small signed integers

[katiehockman]

Approving this cherry-pick since it is a localized, small risk change with no workaround.

[rsc]

Fixed by CL 181978 / 39b533e.