cmd/compile/internal/ssagen: fix misleading comment

Fix up a slightly stale comment in the part of ssa generation that
zeros ambiguously live variables: with the advent of the register ABI,
the ir.Func "Dcl" slice is no longer entirely sorted by frame offset,
although this is still the case for the local vars in Dcl.

Change-Id: I633f43d16f0d4e0b444193a6edb6b2aa1154eea7
Reviewed-on: https://go-review.googlesource.com/c/go/+/316309


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>