cmd/compile: cleanup noLimit in prove

- make it immutable
- calculate it rather than using hardcoded value
  (it all is inlined and constant folded away into noLimit's callers)

Change-Id: Ica12442df4d03e99123e62c0fe561c1078e35d75
Reviewed-on: https://go-review.googlesource.com/c/go/+/727181
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>