cmd/compile: simplify prove pass

cmd/compile: simplify prove pass

We don't need noLimit checks in a bunch of places.
Also simplify folding of provable constant results.

At this point in the CL stack, compilebench reports no performance
changes. The only thing of note is that binaries got a bit smaller.

name                      old text-bytes    new text-bytes    delta
HelloSize                       960kB ± 0%        952kB ± 0%  -0.83%  (p=0.000 n=10+10)
CmdGoSize                      12.3MB ± 0%       12.1MB ± 0%  -1.53%  (p=0.000 n=10+10)

Change-Id: Id4be75eec0f8c93f2f3b93a8521ce2278ee2ee2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/599197
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>