cmd/compile: internal compile error when using sync.Pool: mismatched zero/store sizes [1.13 backport]

[gopherbot]

@randall77 requested issue #39459 to be considered for backport to the next 1.13 minor release.

@gopherbot please open backport issues for 1.14 and 1.13.

This issue can cause the compiler to crash. The compiler was checking an invariant that doesn't, and does not need to, hold.

[gopherbot]

Change https://golang.org/cl/239998 mentions this issue: [release-branch.go1.13] cmd/compile: remove check that Zero's arg has the correct base type