cmd/compile: remove deadcode

Change-Id: Iadf5c4e24440756a4267d2d27f0336c335b5d463
GitHub-Last-Rev: b42f3a5a88
GitHub-Pull-Request: golang/go#78938
Reviewed-on: https://go-review.googlesource.com/c/go/+/770460
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>

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

@@ -52,9 +52,6 @@ func maybeRewriteLoopToDownwardCountingLoop(f *Func, v indVar) {
 	}
 
 	start, end := v.min, v.max
-	if v.step < 0 {
-		start, end = end, start
-	}
 
 	if !start.isGenericIntConst() {
 		// if start is not a constant we would be winning nothing from inverting the loop

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

@@ -23,7 +23,7 @@ type indVar struct {
 	min   *Value // minimum value, inclusive/exclusive depends on flags
 	max   *Value // maximum value, inclusive/exclusive depends on flags
 	entry *Block // the block where the edge from the succeeded comparison of the induction variable goes to, means when the bound check has passed.
-	step  int64
+	step  int64  // it will always be positive.
 	flags indVarFlags
 	// Invariant: for all blocks dominated by entry:
 	//	min <= ind <  max    [if flags == 0]