cmd/compile/internal/ssa: more aggressive on dead auto elim

cmd/compile/internal/ssa: more aggressive on dead auto elim

Propagate "unread" across OpMoves. If the addr of this auto is only used
by an OpMove as its source arg, and the OpMove's target arg is the addr
of another auto. If the 2nd auto can be eliminated, this one can also be
eliminated.

This CL eliminates unnecessary memory copies and makes the frame smaller
in the following code snippet:

func contains(m map[string][16]int, k string) bool {
        _, ok := m[k]
        return ok
}

These are the benchmark results followed by the benchmark code:

goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
                │   old.txt   │              new.txt                │
                │   sec/op    │   sec/op     vs base                │
Map1Access2Ok-8   9.582n ± 2%   9.226n ± 0%   -3.72% (p=0.000 n=20)
Map2Access2Ok-8   13.79n ± 1%   10.24n ± 1%  -25.77% (p=0.000 n=20)
Map3Access2Ok-8   68.68n ± 1%   12.65n ± 1%  -81.58% (p=0.000 n=20)

package main_test

import "testing"

var (
        m1 = map[int]int{}
        m2 = map[int][16]int{}
        m3 = map[int][256]int{}
)

func init() {
        for i := range 1000 {
                m1[i] = i
                m2[i] = [16]int{15:i}
                m3[i] = [256]int{255:i}
        }
}

func BenchmarkMap1Access2Ok(b *testing.B) {
        for i := range b.N {
                _, ok := m1[i%1000]
                if !ok {
                        b.Errorf("%d not found", i)
                }
        }
}

func BenchmarkMap2Access2Ok(b *testing.B) {
        for i := range b.N {
                _, ok := m2[i%1000]
                if !ok {
                        b.Errorf("%d not found", i)
                }
        }
}

func BenchmarkMap3Access2Ok(b *testing.B) {
        for i := range b.N {
                _, ok := m3[i%1000]
                if !ok {
                        b.Errorf("%d not found", i)
                }
        }
}

Fixes #75398

Change-Id: If75e9caaa50d460efc31a94565b9ba28c8158771
Reviewed-on: https://go-review.googlesource.com/c/go/+/702875
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>