cmd/compile: missing bounds checks in 1.11 [1.11 backport]

[gopherbot]

@FiloSottile requested issue #27289 to be considered for backport to the next 1.11 minor release.

@gopherbot please file this for backport against 1.11. This is a regression.

@ALTree please make a cherry-pick CL once your change is merged in master.

[andybons]

CL candidate to cherry-pick: https://golang.org/cl/132495

[randall77]

The fix has been submitted. The "fixed" lines in the CL description were not updated, so this issue was not automatically closed. I will do so now.