cmd/compile: make stack value size threshold comparisons consistent

Consistency is beautiful.

Change-Id: Ib110dcff0ce2fa87b5576c79cd79c83aab385a7c
GitHub-Last-Rev: b8758f8ae02cb025267aa87ebc5c2f9b4c32e742
GitHub-Pull-Request: golang/go#47011
Reviewed-on: https://go-review.googlesource.com/c/go/+/332230


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>