cmd/compile: loopbce cares about the order of successors in the loop header

[Jorropo]

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

Lines 175 to 182 in 6c3b5a2

	if len(b.Succs[0].b.Preds) != 1 {
	// b.Succs[1] must exit the loop.
	continue
	}
	// Second condition: b.Succs[0] dominates nxt so that
	// nxt is computed when inc < limit.
	if !sdom.IsAncestorEq(b.Succs[0].b, nxt.Block) {

This code here assumes the the first successor leads to the loop and that second successor break the loop.
it looks like ssagen always generate the for keyword this way but if you write a loop using if ... { break } inside for { ... } this is usually inverted.

[gopherbot]

Change https://go.dev/cl/685816 mentions this issue: cmd/compile: check domination of loop return in both controls