[release-branch.go1] cmd/gc: accept switches on comparable arrays.

[release-branch.go1] cmd/gc: accept switches on comparable arrays.

««« backport d68605d10a6b
cmd/gc: accept switches on comparable arrays.

The compiler is incorrectly rejecting switches on arrays of
comparable types. It also doesn't catch incomparable structs
when typechecking the switch, leading to unreadable errors
during typechecking of the generated code.

Fixes #3894.

R=rsc
CC=gobot, golang-dev, r, remy
https://golang.org/cl/6442074

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