cmd/compile: update comment for initLimit in prove pass

For: #70156

Change-Id: Ie39a88130f27b4b210ddbcf396cc0ddd2713d58b
Reviewed-on: https://go-review.googlesource.com/c/go/+/624855
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>

Author: fengyoulin

src/cmd/compile/internal/ssa/prove.go

@@ -1569,7 +1569,8 @@ func prove(f *Func) {
 
 // initLimit sets initial constant limit for v.  This limit is based
 // only on the operation itself, not any of its input arguments. This
-// method is only called once on prove pass startup (unlike
+// method is only used in two places, once when the prove pass startup
+// and the other when a new ssa value is created, both for init. (unlike
 // flowLimit, below, which computes additional constraints based on
 // ranges of opcode arguments).
 func initLimit(v *Value) limit {