cmd/compile: don't evaluate side effects of range over array

If the thing we're ranging over is an array or ptr to array, and
it doesn't have a function call or channel receive in it, then we
shouldn't evaluate it.

Typecheck the ranged-over value as a constant in that case.
That makes the unified exporter replace the range expression
with a constant int.

Change-Id: I0d4ea081de70d20cf6d1fa8d25ef6cb021975554
Reviewed-on: https://go-review.googlesource.com/c/go/+/659317


Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>