cmd/compile: improve generated eq algs for structs containing strings

cmd/compile: improve generated eq algs for structs containing strings

type T struct {
    s string
    i int
}

Prior to this change, we generated this equality algorithm for T:

func eqT(p, q *T) bool {
    return len(p.s) == len(q.s) &&
      runtime.memequal(p.s.ptr, q.s.ptr, len(p.s)) &&
      p.i == q.i
}

This change splits the two halves of the string equality,
so that we can do the cheap (length) half early and the expensive
(contents) half late. We now generate:

func eqT(p, q *T) bool {
    return len(p.s) == len(q.s) &&
      p.i == q.i &&
      runtime.memequal(p.s.ptr, q.s.ptr, len(p.s))
}

The generated code for these functions tends to be a bit shorter. Examples:

runtime
.eq."".Frame 274 -> 272  (-0.73%)
.eq."".funcinl 249 -> 247  (-0.80%)
.eq."".modulehash 207 -> 205  (-0.97%)

Change-Id: I4efac9f7d410f0a11a94dcee2bf9c0b49b60e301
Reviewed-on: https://go-review.googlesource.com/c/go/+/230205
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>