cmd/compile: make runtime calls last in eq algs

cmd/compile: make runtime calls last in eq algs

type T struct {
    f float64
    a [64]uint64
    g float64
}

Prior to this change, the generated equality algorithm for T was:

func eqT(p, q *T) bool {
    return p.f == q.f && runtime.memequal(p.a, q.a, 512) && p.g == q.g
}

In handwritten code, we would normally put the cheapest checks first.
This change takes a step in that direction. We now generate:

func eqT(p, q *T) bool {
    return p.f == q.f && p.g == q.g && runtime.memequal(p.a, q.a, 512)
}

For most types, this also generates considerably shorter code. Examples:

runtime
.eq."".mstats 406 -> 391  (-3.69%)
.eq.""._func 114 -> 101  (-11.40%)
.eq."".itab 115 -> 102  (-11.30%)
.eq."".scase 125 -> 116  (-7.20%)
.eq."".traceStack 119 -> 102  (-14.29%)
.eq."".gcControllerState 169 -> 161  (-4.73%)
.eq."".sweepdata 121 -> 112  (-7.44%)

However, for types in which we make unwise choices about inlining
memory-only comparisons (#38494), this generates longer code.

Example:

cmd/internal/obj
.eq."".objWriter 211 -> 214  (+1.42%)
.eq."".Addr 185 -> 187  (+1.08%)

Fortunately, such cases are not common.

Change-Id: I47a27da93c1f88ec71fa350c192f36b29548a217
Reviewed-on: https://go-review.googlesource.com/c/go/+/230203
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>